i ran my 6600 agp with a 300w psu for a while. i had athlon 2000 with 512 ram, 1 80gb hdd, and 2 cd burners, and it ran fine. then i got a new case and now i'm runnin my 6600 with a p4 2.66 oc'd to 3.09 1gb ram 2 hdd's 2 optical drives with lots of fans and lights, but all of that is powered with a 500w psu now.
MacBook 2.2GHz Santa Rosa | 120GB HDD | 4GB ram
Intel C2D E6300 @ 3.2GHz | 4GB ram | 880GB total HDD space | Gigabyte P965-DS3 | eVGA 8800GTS 640mb | Corsair HX 520 | Audigy 2 ZS platnium | Antec 900