If you got a new powersupply, you will have to check to see if it has the right pin layout, as knowing DELL, they would of been HELL and swapped a few pins around.
You will not need 500watts, I run a 6800 off of a 330watt PSU, with no problems. The only problem is that it will put alot of load on the PSU, and decrease its life.
edit: The new cards are big, about 25cm as a rough guide.
Cisco CCNA, Comptia A+, 1/3 through CCNP