@Ricky
Some reviews to check out to help you decide:
Bangs-per-buck GPU shoot-out
http://www.reghardware.co.uk/2008/08/12/review_gpu_bangs_for_buck/
Sapphire 4850 Toxic
http://www.reghardware.co.uk/2008/09/13/review_sapphire_4850_toxic/
AMD Radeon 4850 and 4870
http://www.reghardware.co.uk/2008/07/23/review_amd_4850_4870/
GeForce GTX 280
http://www.reghardware.co.uk/2008/07/09/review_geforce_gtx_280/