Will my AMD Phenom II X4 955 overclocked to 3.5GHz run well with a GTX 660
I have DDR2 800MHz RAM installed aswell. I have tried my friends 7850 in my PC and I only say about a 5-10 fps drop compared to his i5. Would a GTX 660 work well?
The GTX660 is slightly stronger than a 7850, but not so much that you could tell a difference without a synthetic benchmark.
If you liked the performance with the 7850, you will like the GTX660 just as well.
Most games are limited by the graphics card, and not so much by the cpu.
Your X4 was a very good chip in it's time, and is still decent today. But, the fact that you saw a fps drop compared to an i5 indicates that your games may be somewhat limited by your cpu.
If a GTX660 is a big enough jump over your current card, I say go for it.
If it is only a smallish jump then you might rethink.
It depends on what games you play and where you notice slow down. I moved up from a 5770 to a 6870 and then a 7850 in my 965BE@3.8Ghz. The 5770 to 6870 was a huge leap, and in the games I play (like Planetside 2 for instance) I couldn't even measure a difference in my frame rates when i went to the 7850, so at that point the CPU was just too slow to keep up with PS2 anyway so the extra 15-20% of the 7850 wasn't even notice-able. Though the 7850 was a hand-me-down card from my main machine that went to a 670, so it's not like I was out anything.
The point though is that you can do things like crank up anti-aliasing or other GPU heavy tasks when playing and it won't impact performance as much since the video card is waiting around on frame data anyway. You just have to go in to it knowing that it will probably perform similarly to the 7850 or even the 6870 even though it's a more powerful card.
Though in cases where CPU doesn't matter as much, it will still perform better than lesser cards anyway . . . just as long as you are aware of the CPU limitations going in so you don't feel disappointed when some games still chug when CPU demands increase.