Desktop Parts

1. CPU
	1.1 Frequency
		One of the most obvious choices, higher frequency (core clock) means better performance, granted other variables are the same. 3GHz is typically the standard for a high-end computer, so long as it has the cores to suffice.
	1.2 Cores
		The other major factor in deciding what CPU to get, barring price. Generally, the more cores, the better, but things get complicated. Until recently, games never used more than one core, and still there are quite a few that get hardly any boost in performance when upping from 2 to 4 cores. Getting a 6-core, on the other hand, is generally absolutely useless except for a select niche of games that utilise them; typically these games have high amounts of large-scale algorithms, or many sophisticated bots. More cores mean more raw power at the expense of more heat, more heat means need of better cooling (fans, heatsink, case).
		1.2.1 Application
			Two cores is usually good enough for a budget gaming computer, but is certainly plenty for any normal desktop user, yet is not nearly good enough for a serious gamer who wants to squeeze every ounce of gameplay out of a video game. For these, a quad core is recommended.
	1.3 Manufacturor
		At this point, the only real deciding points are 1. Price and 2. Compatibility. If you have an Intel motherboard, get an Intel, of course, and if you are penny-pinching, get and AMD. There are a very slight few differences between the two hardware-wise, so I would always recomment AMD.
		1.3.1 Intel
			As you must know, these buggers are everywhere, used for every application, and are leading the CPU race. They are also twice as expensive. Why do people choose them? Many report to have better stability in some circumstances, but that can also be applied to AMD. The biggest surface difference, as I have witnessed, has been the stock core clocks.  Many Intel chips ship with 3.3 or even 3.5GHz clocks, with perfectly fine stability, making them very attractive to those who wish not to deal with overclocking.
		1.3.2 AMD
			Much more wallet-friendly than their competitor, Intel, these are the choice pick for many users for reasons ranging from user-end dedication to ease of overclocking to the simple price factor. More tech-savvy computer builders quickly note that the sockets that AMD use are very cross-compatible, and HSF (heatsink/fan) universal. These processors tend to ship at lower core clocks (2.8-3.2, generally) but
		1.3.3 IBM
			If you have to ask, then you shouldn't even think about getting their processors. Otherwise, go right ahead!
	1.4 Run-Down
		While choice of manufacturer is subjective, other specs would be:
		1.4.1 Light desktop user:
			>=2.4GHz Dual core or >=3.0GHz Single core (if you can find any)
		1.4.2 Light gamer:
			>=2.6GHz or >=3.2GHz Single core
		1.4.3 Mid-range gamer:
			>=2.8GHz Dual Core or >=2.4GHz Quad Core
		1.4.4 High-End gamer:
			>=3.2GHz Dual Core or >=2.8GHz Quad Core
		1.4.5 Enthusiast
			>=3GHz Quad Core or >= 2.6GHz Hexa-core
		1.4.5 3D Graphics designer
			>=3.2GHz Quad Core or >=2.8GHz Hexa-Core
		1.4.6 Game Server hosting
			(See mid-range gamer)
2. RAM
	2.1 Type
		The socket of your CPU and motherboard is the main deciding factor of this, but granted you are making a brand new computer, just get DDR3, no questions asked.
	2.2 Capacity
		The primary deciding factor in the ability of RAM, but completely subjective. Generally, aim for 4GB. See run-down for more precise recommendations.
	2.3 Channel
		Single vs. Dual; What's the difference? If you have 1 stick or 4GB RAM, there is a marginal increase in performance over 2 sticks of 2GB RAM, granted other variables are the same. Having to move data between the two sticks increases latency, decreasing performance.
	2.4 Frequency
		The second most important factor, also subjective. Generally, aim for 1600MHz. See run-down for more precise recommendations.
	2.5 Timings
		The most obscure and complex factor about purchasing RAM - CAS Latency. Column Address Strobe Latency is just the delay between each operation performed. The numbers are portrayed in a 4-number sequence, I.E. 7-8-7-29. For all general purposes, the first number is sufficient for selection. Since we are dealing with latency, the lower the number, the better. As RAM as gotten bigger and badder, however, not all aspects have improves. You can never find a decent stick of RAM with less than 5 CAS Latency, when before 2 was more-or-less frequent. Even so, the number is very pertinent to purchasing RAM. Generally, aim for 8, but 9 is perfectly OK in any situation. See run-down for more precise recommendations.
	2.6 Manufacturor
		This one's a doozy. I'll just list the companies you should worship, which you should avoid, and whatever else may be relevant. Will only deal with gaming-level companies; anything less is pretty much a toss-up.
			Corsair - Best company out there, for a variety of computer parts. They know this, though, and will charge you extra for it. If you get a chance, jump on it!
			Patriot - Perhaps the RAM runner-up, also a very dependable maker.
			G. Skill - General gamer's choice; perhaps the best balance between performance, make quality, and price, a good find no matter what.
		Other recommended companies:
			A-Data, Crucial, GeIL, Mushkin, PQI
		Companies you should generally avoid:
			OCZ - The list of DOA (dead on arrival) and otherwise defective RAMsticks that come out of these guys just does not end. When the stick is good, however, you can be assured that it is perfectly fine, even optimal performance RAM. However, the chance is just too risky for many.
		Any company not listed here is generally a decent go if it's price is right.
	2.7 Run-Down
		1.4.1 Light desktop user:
			2GB 1066, CAS 9
		1.4.2 Light gamer:
			2-4GB 1333, CAS 9
		1.4.3 Mid-range gamer:
			2-4GB 1333+, CAS 9
		1.4.4 High-End gamer:
			4+GB 1600, CAS 8
		1.4.5 Enthusiast
			8GB 1800, CAS 7
		1.4.5 3D Graphics designer
			8GB+ 1600+, CAS 8
		1.4.6 Game Server hosting
			8GB 1600+, CAS 8
3. Motherboard
	3.1 Socket
	3.2 Size
	3.3 PCIe x16 capabilities
	3.4 Ports
		3.4.1 SATA/PATA
		3.4.2 PCI/PCIe
		3.4.3 Fan
		3.4.4
	3.5 Onboard
		3.5.1 Video
		3.5.2 Audio
		3.5.3 USB/eSATA
		3.5.4
4. Video Card
	4.1
5. Hard Drive
	Note: Hard drives only, no Solid States! Look them up yourselves! For a general user, they are too expensive for their own good. Once their price goes down, they will become an adamant figure of storage choice, but as of now they just aren't worth it for the vast majority of users. If you want to shell out money for the speed, though, go right on ahead, though.
	5.1 Capacity:
	5.2 Speed:
	5.3
6. Power Supply
7. Case and Cooling