I want to say just get a new GPU and wait, but its a dell. To get a decent GPU you need a better PSU. Dell cases aren't known for their great cooling or air flow either so that could cause issues. And a Dell prop. mobo... eesh.
If you can wait, try to till the new 2011 SB parts and BD release at least, to get existing parts cheaper at least.
If you want a whole new system keep the CPU, optical drives, HDD's and Sound card at least.
Basically it comes down to what you are doing and what you plan to do in the future, knowing that would help us provide the best advice, otherwise, we can pull you in a million directions