i was planning to get teh x1900aiw onw with thermaltake purepower 480W PSU and to be honest i placed many posts on different forums just to make shoor that it will work, in most cases i was told that I would be fine, but in You r case if it saiz it needs 450W which im shoor it does, i would not get a lower Watage PSU, (n my opinion i think its a waste of $$) it might not work, and for shoor it will strain the psu, btw have You checked the Amperage on the psu ? i think that ur card needs a minimum 30A on a 12v rail ( my psu has only 18A but it has 480W, and to be honest i think that ill have to buy a new psu )
Don't forget to add your CPU and your RAM to that list, as well as any PCI cards.
Last year, I built a PC for my brother with an eVGA 7800GT. The specs called for a 450w PSU, and stupid me assumed that his Antec case came with one. Unfortunately, it only housed a 400w PSU, and he was occasionally getting BSOD'ed in WoW at 1050x1680. His PC was pretty simple: AMD 64, 2x512mb Corsair VS DDR, 200gb DMax 10 (7200rpm), Asus A8N-E mobo, Audigy 2 Value sound card, and a Plextor DVD burner (he hadn't started burning anything at that point). Luckily, I caught my mistake very soon, and we swapped it out with a 500w Antec PSU. Aside from the fact that the first 500 watter was a dud, he's had no problems since.
So you could probably get by with a 430w PSU, but if you start experiencing any problems, upgrade immediately. The last thing you want to do is to burn out a $400 videocard, when a $100 solution would have prevented disaster.
Oh, and one other note: pay attention to any warnings you get from your mobo. When my brother's PC was first put together, I added a second fan which would cause the mobo to issue warnings about the CPU not getting enough juice. For the time being, I just unplugged the second fan. Cocky me should have paid attention. With the 500 watter, we could run everything with nary a hitch.
One more thing: unless you're REALLY sold on the XTX, you might want to get the XT instead. The speed difference isn't huge, and you could always OC your XT to XTX speeds. I'd only get the XTX if the price difference was $30 or less. At the time I purchased my XT, it was $40 or more (oddly, the retail version offered a rebate that made it cheaper than the OEM version).