Huh? You are going to need to be a bit more specific, what parts were you thinking about upgrading?
XFX is pretty solid in all of their offerings, but the other ones offer far more variety. Take antec for example, the true power and earthwatts series are good but the low end basiq PSUs are crap. We cant recommend by brand, if you want suggestions for components head over to the homebuilt section and make a post using the template shown in the sticky.
I'll have to respectfully disagree with hunter on the Basiq. The 430W model is only mediocre, but the 500W version received a very good review at hardwaresecrets. Still, a 500W PSU would be a minimum for this build, and a good 550W model like a Truepower New would be better.
In addition to Antec, other good PSU brands include Corsair, Seasonic, PC Power & Cooling, and Enermax. Coolermaster and Thermaltake are hit and miss; some ok to good, others (especially older ones) are crap. OCZ isn't crap, but is definitely not in the same league as the first five I mentioned.
The absolute most popular psu brand on thg seems to be corsair (and for good reason). The others mentioned are still top quality and would work great. Just stay away from those generic brands and watch the efficency.
+1 for the 5870/5970. The GTX295 is massively overpriced compared to these cards. Not to mention the lack of dx11 and eyefinity.
One things for sure... you are going to have one beastly machine.