The system should be pretty good with the specs listed, Vista is definitely not the best version of Windows to be trying to run games from. The Quad Core CPU is more than adequate, the SLI VGA is probably not the issue, 2Gb of RAM should not be the issue either, so that leaves perhaps Vista (quite likely) or your hard drive. Try something as simple as run the disc clean up and then de-fragment your drive, both can be found by opening the Computer Icon, right click on the C Drive, select Properties, on the main page is the clean up tool option, on the Tools tab is the de-fragment option.
I would suggest a few changes, you can try and over clock your CPU but it is not the easiest of things to get right if you have never tried it. You need a mainboard that will allow it, usually a better cooling option as well and a bit of trial and error.
I would also suggest you upgrade to Windows 7, check the W7 upgrade adviser on the micro$oft website and make sure your hardware AND software are compatible. The better option is the 64 bit version as it allows the CPU to use its full potential but you have to be sure your other hardware is 64 bit happy, printer, web cams, anything you plug in to the PC basically.
You can not upgrade from 32 bit to 64, it has to be a clean (new) install which means you have to back up your old data, wipe the drive, install W7 64 bit and then reload all your programs again. The bonus is you get rid of all the inconsistencies in your old set up and even with just 2Gb of RAM it will be faster than the Vista set up.
You can give the machine a bit more zoom by adding more memory in its current 32 bit Vista configuration, but ideally you want just 1 extra Gb as the 32 bit version of Vista (or W7) will not recognise beyond 3.57Gb of RAM and probably less with your two VGA cards in SLI. With the 64 bit you can use four and more gigabytes of RAM which is another good reason to install it.
FYI - other answerers are probably correct, your CPU is most likely the Q6600 and the OC that was referred to when you added the additional details is not a different type of CPU but simply means Over Clock, the answer simply suggested change it or Over Clock it.