4850 power draw is much lower than Nvidea cards. 500W psu should be more than ample. You will be extremely happy with this card. The performance of 4850 was close to Nvidea 9600GT(better in some areas and less in others). I believe 4850 was better with 3d graphics over 9600. Hopefully ATI have the driver issues figured out. From 3870 and up the cards were running warm but they all should now have a bios update. I had 2- 3870 (512)crossfired cards and benchmarked 3dmark06 at 16,000. with 1 card I was benching 13,000 which 4850 would be higher. I ran AMD 9850 quad core with the 3870's when I benched.
Eagler. Rivatuner can be used on ATI cards but are limited in settings. As said abouve fan speed can be adjusted on ATI cards using Riva.