Depends a lot on your budget. Even an 8600GT is going to be a stretch with a 305 watt power supply. Looking at Dell's pics online, it looks like a pretty standard sized power supply. So you could easily stick a 500-750 watt unit in there and go with an 8800-series card (maybe an 8800GT with an extra cooling fan for safety).
Power supplies are easy to swap out, but some Dell mobo's use a proprietary connector. However, you do not necessarily need a Dell power supply, as I've seen adaptors for standard mobo connector cables, so any ol' ATX-form power supply will do, just add a cable adapter into your budget (
http://www.endpcnoise.com/cgi-bin/e/...=dellconverter ).
P.S. On ebay, it seems that even Dell makes (or brands) a 600 watt power supply for that model of computer - I saw one ebay store with them for a buy-it-now price of $25.00 US. Personally though, I'd spend a bit more and get a better quality 3rd party power supply - OEM power supplys tend to be of the cheaper (price and quality) kind. Web stores like
www.newegg.com and
www.tigerdirect.com have plenty of good brand power supplies in the 500 watt range for $40-$80.