Try bumping it up about 300mb at a time, and see if it makes a difference....if it still lags abit, move it up higher...til you find a value that works for you.
You could also try upgrading your Windows XP to Service Pack 2 and the next time you run an old game or CS right click on the shortcut select properties then select the compatibility tab and choose to run the program in Windows 98/Me compatibility mode.
