I know these cards need a 400watt supply but I read in a lot of forums that a 300watt supply can do the job too. So i'll just drop in my specs and let me know if i can run any of these cards. I am using a Dell E521.
I have a :
AMD Athlon 64 x2 3800+
2 x 512 MB RAM (NANYA - http://www.nanya.com)
Max power output 305 watt supply (DELL)
Seagate 160GB SATA HDD @7200RPM + 500 GB Transcend(External) @ 5400RPM
I think that about it.If you want any more details let me know I'll search and let you know.
A 4650 would work easily, doesn't require external power, a 4670 would be a better option that still doesn't require external power.
2 type's of 9800, the reference card that needs a 6 pin connection and the Eco or Green card's that don't.
Either would work, the standard would be a bit harder on the psu and needs you to use an adapter or two.