For performance to be equal you would probably need to overclock the 7900GT, but keep in mind the X1800XT can be overclocked very easily with software voltmods. However if you do overclock you would need an aftermarket cooler on either card.
The 7900GT does run somewhat cooler though and probably comes with a better warranty.
I personally had to decide between a 7800GT and a X1800XL(I chose this) and have not been disappointed. The IQ (FP16 HDR+AA can be done on ATI X1K cards while it cannot be done on NVIdia cards) is great and almost all games (Tomb Raider Legends with next-gen content on being the exception) run extremely well. If your brother is planning on playng Oblivion, I would definitely suggest the X1800XT. The ATI cards seem to do very well in Oblivion.
Do you have links to the specific cards you were thinking of?? Don't forget to search through the forums, there were several of these 7900GT vs. X1800XT threads made.
Originally posted by: Aflac
X1800XT is still faster at stock than a 7900GT - unless you plan on overclocking, in which case the 7900GT walks all over the X1800XT. Also, 7900GT consumes less power and runs cooler, generally.
You can also overclock the X1800XT though and is much easier to do so through software, whereas the 7900GT requires a volt hard mod to overclock significantly and would still need an aftermarket cooler just as the X1800XT would if overclocked.