I think you are going to need these two files.

Assuming you are running Windows…

This one will revert your phone to a different stock OS than you have.  This one is more compatible with ROM Manager from what I have read.  I still had some issues but was able to plough through with it.  In order to use this:

First make sure you have titanium backups of course.

1.	Start the application below.
2.	Turn your phone off.  Then, while pressing both volume up and volume down, connect the USB cable while connected to the computer.  This puts the phone into download mode.  The app will indicate something like “ready”, or “started”.  Click Start, watch the progress bar and wait for about 5 minutes.  If it seems stuck after 5 minutes, yank the battery, close the app, and start the process again.  I always have to do this twice on my phone for it to go through.
http://dl.dropbox.com/u/15227809/I897UCJF6-final-OCD-REV0.exe


3.	This one will root the above stock OS once you have flashed it.  Boot up phone, connect it, run the below.  After done, on the reboot, choose “reinstall packages” from the Samsung recovery menu.
http://dl.dropbox.com/u/15227809/Galaxy_One-Click_Root.exe


4.	At this point, in theory, you can download Rom Manager Premium from market, and flash “Recovery”.  This is where it got weird for my phone.  I had to mess with this, rebooting to recovery, choosing to either reinstall packages or reboot phone.  EVENTUALLY, I get the real clockworkmod recovery menu that is needed.

From here, I think I could still not use ROM Manager to install Cyanogenmod.  From the phone, I downloaded nightly 17 build from here http://download.cyanogenmod.com/?device=captivatemtd

Then, using ROM Manager, choose to reboot to recovery, and select zip to install.  Navigate to /sdcard/download (or /mnt/sdcard/download) and choose the cm_captivatemtd_full-17.zip you just downloaded.

Let it install, the on reboot, re-download Titanium from market with pro key, and reinstall all your programs, reconfigure everything like your home screen and email, and now you are done!

After this, in theory it is easier.  For example, to get from the prior 16 ROM I was on, I just got a notice from ROM Manager that a new one was out, and it does everything for me.