I've got 2 78gtx's... one 88 series card, from what i've heard, is more powerful than 2 of mine..... plus, it will be dx 10 capable for future games.
an in terms of power supply, yes ,you'll be wanting a beefy one, i'd say at least 500 watts.
And powersupply brands matter btw, antec seems to be good, but if you want one that will never let ya down, go for a pc pwer and cooling psu. my 510 watter runs an fx-57, 2x7800gtx's, 2X sata 3gb hd's, an ide hd, a sata dvd burner, a cd rom drive, piles of fans, and all types of other crap. never so much as a hiccup. they cost a bit more, but you get SO much out of buying one.