Well, as far as I know, Dell computers (not including their top end XPS line) generally ship with less than adequate PSUs. If you're talking about replacing your graphics card with something thats of similar power, sure, you can just pop it in without worrying a thing.
But here, you're talking about the 2nd biggest graphics card out there at the moment. And unlike the biggest CPU range out there, these are pretty much power hogs where graphics cards are concerned. My bet would be that you'd need a new PSU to run an 8800GTS without problems. I'd sleep better too.
A new PSU that'll perform pretty well sould set you back about 100-150 bux. Shouldn't need to look into anything above 600W. I'd recommend something at least 500W, but that might be cutting it abit low.
I'm building a new comp, and I'll be getting myself an Antec TP3 650W, and I'm pretty sure that should cover an 8800gtx if I needed to upgrade.