Your monitor is missing from the rig in your sig. Not only is it the most important piece of hardware in your system (you stare at it pretty much the whole time you're using the computer), but it's also the most important piece when determining if there will be a CPU bottleneck for your GPU because it determines what resolution you (can) play at.
The type of game plays a large role as well... for example, RTS games perform better with faster CPUs even at high resolutions such as 16x12. And there are other games, such as FPSs, that go the other way-- a faster CPU won't even make a difference at 1280x1024.
In general though, your X2 will not be bottlenecking the GTS. There might be a particular game (such as Supreme Commander) where a faster CPU will make a significant impact no matter what resolution, but the 8-series cards a soooo much faster than your 6600gt that no level of bottlenecking will take away that "holy crap" feeling you're going to get once you install it.