You plan on getting one of the best video cards out with that set up? I dont think so. I have a Pent 4 3.0ghz, and a gig of ram, with an eVGA 7600GT, and im still getting bottlenecked somewhere. Might be the 375w psu, even though im only needed to have a 350w one.
Im not sure, but it doesnt matter. What im trying to say is that with your computer, it will bottleneck like none other. Try for a 6800GS. That or a type of X1600XT. Or even new mobo, and get a dual X1300XT 512mb version. That would be cool. Though only 4 pixle pipelines each. But for like 130 or less each, not a bad deal.
As for your brother, what is he running?