V-Sync forces your over all FPS count to limit itself (to be in sync with) the Refresh Rate of your monitor. For example, MOST monitors have a 60Hz refresh rate. So with V-Sync turned on your FPS will MAX at 60FPS. With an ATI 5850, you can turn on V-Sync with some games to reduce 'overall' FPS, however gain a crisper image.
Basically, a 60Hz monitor cannot display more than 60FPS anyhow. So by using V-Sync your system will choose the BEST images to display in that 60FPS, rather than whatever happens to get shown. This usually gives you an image that's more stable, and can remove 'tearing' issues if you have graphical anomalies in some games.
However, if you just want to see how many FPS your video card is capable of in a particular game, turn V-Sync off.
Games typically DO NOT use hyperthreading, so for that it's basically pointless. At this point, the most common applications that use hyperthreading are Video/Photo editing software, some zip/unpack utilities, and other such applications. So far as games go, they don't hyperthread well. So the higher your CPU clock speed, the better.