I would get the HD 5750 instead of the HD 4850.
1. It uses less power.
2. Same price, same performance.
3. DX11 card.
The Corsair should be capable of powering the HD 4870. It is more expensive than the HD 5750, but less expensive than the HD 5770. It also a little more powerful than the HD 5770, but is a DX10 card. True DX11 games will probably be out in about 6 - 9 months.