I've got a Dell 8300 that came with a 9800pro and 250 watt ps. Granted, Dell's power supplies are underrated and tough as nails, but if your current ps has an extra plug, chances are it'll run fine.
As far as 128 vs. 256, I read an article, I think it was on tomshardware.com where they benchmarked the two and there was very little difference.
This will blow away your current onboard video. I went from a gefore4 ti 4600 and this is about 2x as fast with the new games that utilize DirectX 9 stuff. A little faster with the older games, but for games like farcry and painkiller, this card is pretty much the bomb. (Unless you're willing to spend twice as much for an X800 or Geforce 6800)