You'll need a new PSU if you upgrade the video card.
Since its a Dell, you cannot overclock. Dells makes the BIOS so you can't.
Your CPU is probably fine, but if you must, I'd go for something like an E8500 since you cannot OC.
Your GPU is the problem here. 9800GTs and 4850s have both come down considerably in price and are leaps and bounds faster than what you have at the moment.