I think a lot will depend on your budget. If it is limited, and not likely to grow much, then putting $59 into a dual-core S939 chip like this one:
AMD Athlon 64 X2 3800+ Manchester 2.0GHz 2 x 512KB L2 Cache Socket 939 89W Dual-Core Processor - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103561
...will make a difference. You'll offload lots of background crap and really smooth out your gameplay. Even though maximum frame rate may not change [much], the minimum frame rate almost certainly will, reducing or even eliminating stutter. Note that this is an OEM chip, so it won't come with a cooler. You can probably re-use the one you have, provided you clean off the thermal paste and re-apply new paste.
Yes, you will need to load the AMD multicore driver, available from their web site (or Google for it). A Windows re-install should not be needed.
If, otoh, your budget allows a more substantial upgrade, there is no reason to spend this $59. With a limited budget, you may still want AMD, but you'll get best performance these days with Intel. Please tell us your budget.
I don't have Crysis, so I can't comment on that, but have the general impression that SLI / Crossfire aren't worthwhile. Once again, it will depend on your budget, but a single good card will perform better than your pair of old 7800GTs.
Another thought on performance. Assuming that 36GB Raptor is the only drive in your system, how full is it? If it is so full that Windows can't maintain a decent swap file, you'll take a serious performance hit.
Also, what is your RAM configuration?