At 1280 X 1024, the GTX+ will be somewhat better. That PSU is a bit itchy for either card though. I'm not too sure of its quality, so it may be underpowered. The 4850 is very close to hanging with the GTX+ at this resolution + AA. If you up the AA or res, you will see the 4850 having an advantage at most games. Crysis is an exception to this.
Ultimately. 9800GTX+ has more stable drivers, comparable performance at low res, and a decent heatsink.
for the same cost you can get An non-Oem 4850 which has better overall performance, and dx10.1+tessellation going for it
Yeah, I'll probably upgrade the PSU to atleast 500/550/600W.
But the thing is.. I'm looking for the long run. I don't usually upgrade my GPU often - I'm still using a 7600GT with great success. That's why I love Nvidia and feeling slightly hesitant switching to an ATI card.
I heard it runs at really, really hot temperatures, too.
The 4800 series running at hot temps was due to poor stock heatsinks / low fanspeed. This version of the 4850 will run around 50 degrees at load overclocked, provided you have decent air circulation in your case. Unfortunately you will be sacrificing more homogeneous performance across the board in exchange for ATI's constantly updated (but not always perfect) drivers.