X1950 Pro vs. 7950GT Power Requirements

Hey, I'm new here as far as posting goes but I read these forums alot and this seems to be the best group of people to get advice from. I've read post after post regarding the X1950 Pro and power supply requirements, but every pc is different so I was looking for some information regarding my rig specifically. The X1950 Pro seems to be the best perfomance/price card out there right now and my budget is about $160 on a new card. I've only used Nvidia in the past but moving to ATI for this price range seems in my best interest. Also my psu, I feel, is borderline to use with this card but I'm looking for a second opinion.

I currently have a Rosewill R550S-2M psu, 550w, 420 max on the 12v1 and 12v2 rails. They are rated at 18amps a piece which seems enough...maybe. Cooling wise I feel that my Coolermaster Ammo Box, with 1 intake, 2 rear exhaust, and 1 ceiling exhaust fans will be enough, they are all LED if that has any impact on amps at all. The rest of my setup is below along with my soon to be upgrade. Any advice from experience with this card is greatly appreciated regarding my psu w/ my current and future system.

CURRENT
AMD Sempron 3100+
FIC Mobo
1 gig DDR400
1 x DVD/RW
1 x CD/RW
1 x Seagate Barracuda 250gb HD
SB Audigy SE sound card
SMC Network card
Front Bay LCD Display w/ temp etc.

UPGRADING TO
AMD X2 4400+
ASUS single PCI or Gigabyte Mobo (unsure yet of exactly which one)
2 gigs DDR2

everything else will be the same.

Thanks in advance.

Your psu has 35amps so your good there. I second the 3850 and the 5000 (I have the 5000 myself but not the black ed. can't o'c mine)