Nice choices but I would at least get a bigger hard drive, the cost per gig get cheaper and cheaper as you get bigger, I say at least 500GB but aim for 1TB.
Also do a double check with that power supply... 550W for a gaming rig is a little low... power is too important to scrape by with especially with gaming
The motherboard you chose has 2 PCI slots; I would look for something with only 1 PCI slot since PCI is on its way out... so it will be wasted space on the board… I would get a board with at least 2 PCI-2.0x16 slots so that you can plan on expanding for the future. No sense on paying money for 2 boards, the one today and the other one tomorrow.
And in regards to your video card... nice choice but you’re better off getting nVIDIA cards from eVGA, BFG or XFX... they are the only card manufactures that provide lifetime warranties on the video cards…
The card you chose is
Manufacturer Limited Warranty period (parts): 3 years
Manufacturer Limited Warranty period (labor): 2 years
The card manufactures I mentioned above have lifetime limited warranty
Besides ATI blows... no sense in buys 2 cards, the card today and the one you have to buy to replace it.