Looking for advice about buying a new GPU

I've been thinking it's about time to upgrade my GPU but I haven't been paying attention to hardware for a few years since I built my last PC. My goal is to try to bridge the gap between my next full PC build.

Here are my current specs:
8800GTX, Q6600, Gigabyte GA-EP45-UD3P, 4GB RAM, PCP&C Silencer 750W, Antec 900

I run games at 1680x1050 (also have a secondary monitor at 1280x1024). I've found that I can no longer run newer games with the settings on high at the smooth framerates I'm used to. Games I've been playing lately include World of Tanks, Battlefield: BC2, Borderlands, Left 4 Dead 2, Team Fortress 2, Heroes of Newerth. I don't really care about AA at this high of a resolution, but I do like to crank the rest of the settings up.

First, I am trying to determine whether upgrading my GPU would give me a significant performance boost. Would my CPU be a bottleneck here? I'm currently running at stock speeds but I know I can safely OC it to at least 3 Ghz if it would help (I already tried and didn't notice much if any fps gain).

I am thinking of buying the new GPU in the next couple weeks and am hoping to spend around $200. That's not a hard limit - I could go higher if necessary - but I'm just looking for something to keep me going for maybe a couple years until I decide to build a new PC from scratch.

I've tended to go with nVidia in the past, but this mobo supports CF and not SLI so it might be worthwhile to go ATI and leave that option open in the future? On the other hand, is PhysX something worth with staying nVidia over?

I appreciate any thoughts you guys could share with me about my situation. Thanks!