Upgrading from an 8800gt

Hello, folks.

I have been running a Zotac 8800gt 1gb for about a year now. I mostly play WoW, which doesn't really tax cards lower-end than my dated card but still, every now and again, I will dip down in fps from the standard 60 to the 20s then 30s and them back again to 60 (with or without addons). It makes my skin crawl, I get so mad. I also occasionally play a FPS (COD or something along those lines).
Here is what I'm up against; I'm looking at the GTX 200 line. I am looking at the top 3, the 275, 285 and, of course, the 295. I don't mind shelling out the dough for good stuff if it's really "the &@*#! But if I surely don't want to shell out an additional 200 bucks for "eh, I can notice some difference."
Like I said, I am coming from a virtually no-name 8800gt so any of these will rock, I'm sure. But if I need a single-slot gpu solution and will probably never be playing on anything greater than 22" monitor, will I still be way ahead to shell out the bucks for the 295 over either the 285 or 275?

Thanks for your time

Hemphill

Power supply is immaterial as I will have to upgrade regardless of which one I choose

Intel E8400 (bumped up just a hair)
OCZ Reaper 1066 2gb
XP Home
Biostar TP43D2-A7 (it was cheap and has never given me trouble )
Currently just using onboard sound (Creative and Nvidia don't play well together, in my experience...dunno why)
currently only running 19 wide monitor (not particularly high res)