Unfortunately, Dell uses proprietary motherboards, power supplies, and cases. If you wanna upgrade one out side of Dell's productline, you need to upgrade the other two.
Sooo, you'll have to either buy a new motherboard and case, or buy a power supply through dell, or find one on, say, eBay.
For now, you will need to keep that 6600GT out..so you don't damage it and/or the rest of the system.