FooBar.xml
FooBar(1).xml
FooBar(2).xml
...
FooBar(N).xml

var filenameFormat = "FooBar{0}.xml";
var filename = string.Format(filenameFormat, "");
int i = 1;
while(File.Exists(filename))
    filename = string.Format(filenameFormat, "(" + (i++) + ")");

return filename;

/// <summary>
/// Provides a filename given if it does not exist.
/// If the filename exists, provides the lowest numeric number such that
/// filename-number.ext does not exist.
/// </summary>
public static string GetNextFilename( string desiredFilename )
{
    // using System.IO;
    int num = 0;
    FileInfo fi = new FileInfo( desiredFilename );

    string basename = fi.FullName.Substring( 0, fi.FullName.Length - fi.Extension.Length );
    string extension = fi.Extension;

    while( fi.Exists )
    {
        fi = new FileInfo( String.Format( "{0}({1}){2}",
            basename,
            i++,
            extension ) );
    }

    return fi.FullName; // or fi.Name;
}

public static string GetNextFilename(this string filename)
{
    int i = 1;
    string dir = Path.GetDirectoryName(filename);
    string file = Path.GetFileNameWithoutExtension(filename) + "{0}";
    string extension = Path.GetExtension(filename);

    while (File.Exists(filename))
        filename = Path.Combine(dir, string.Format(file, "(" + i++ + ")") + extension);

    return filename;
}

string fileNameFormat = "FooBar{0}.xml";
string fileName = "FooBar.xml";
string filePath = "C:/";
string[] existingFiles = Directory.GetFiles(filePath, "FooBar*.xml");
int i = 1;
while (existingFiles.Contains(filePath + fileName))
{
    fileName = string.Format(fileNameFormat, "(" + i + ")");
    i += 1;
}

Dictionary<string, int> fileNameOccurences = new Dictionary<string, int>();
// ...
string fileName = "FooBar";
if ( fileNameOccurences.ContainsKey(fileName) ) {
    fileNameOccurences[fileName]++;
    fileName += "(" + fileNameOccurences[fileName].ToString() + ")";
}
else { fileNameOccurences.Add(fileName, 1); }
SaveFile(fileName + ".xml");

string fileName = "FooBar";
string[] fileNames = Directory.GetFiles(theDirectory, fileName + "*.xml");
fileName += "(" + (fileNames.Count + 1).ToString() + ")";
SaveFile(fileName + ".xml");

string fileName = "FooBar", directory = @"C:Output";
int no = 0;
while ( ++no > 0 && File.Exists(Path.Combine(directory, fileName + "(" + no.ToString() + ").xml")) );