The 9800GTX is basically a G92 (512MB) version of 8800GTS. The 9800GTX "+" is basically a beefed up or overclocked 9800GTX. Not a super huge difference. Though if you're buying new, I'd get the GTX+ as it is faster.
The ATI 4870 though, is definitely the better of the two video cards. As far as trying to keep yourself from upgrading again too soon, I'd suggest the 4870 if you don't mind the extra cash. It'll impress for sure.
Personally, I could care less whether I'm running nVidia or ATI. Both make very good cards. Right now though, in your price range, ATI has the edge.
I'll second the motion to upgrade your CPU too. That AMD 4200+ is fading out quickly. You'll see much better performance with something newer.
If you're going to go quad core, I wouldn't settle for the current Phenom quad cores. At least bump up to the Intel Q6600, rock solid CPU and overclocks easily on most Intel based motherboards.
4GB of RAM will make a visible difference.
Quad Core CPU will make a visible difference.
4870 will make a visible difference.
If you decide to stick with the 9800GTX+, get the one from Tiger Direct. Looks like a good deal.