| You might not even need that high, I'm only running off a 400W PSU that came in my case (my system specs are in profile) so it's up to you, you might need more than me as the 6600 is newer than my card, and the CPU is clocked slightly higher, but you shouldn't need more than about 450W i wouldn't have thought.
EDIT: Opps, didn't notice this is over a month old, I just used the related forums links at the bottom of the page
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My System: First OC Processor(s): Intel E2180 @ 2.85 | Motherboard: Gigabyte GA-P35-DS3L | RAM Memory: 2x1GB OCZ PC2-9200 reaper CL5 | Graphics Card(s): Gainward ATI 3850 | Sound Card: on board | Hard Drive(s): Seagate Barracuda 7200.7 120GB | Optical Drive(s): HITACHI DVD-ROM GD-2500 | Case / PSU: Corsair VX450 | Cooling: AC freezer7 Pro, 2x80mm, 1x90mm, 1x120mm | Network / Internet: on board / supposedly 10Meg virgin cable | Monitor(s): Viewsonic Vx922; Viewsonic VE702m | Operating System(s): XP Home | |