Whats the PSU? What games do you play? At what res? Budget?
The 5670 sounds great, but make sure your computer can handle it. We need specs.
The games I play most would be GTA IV and Black Ops. Dell calls the PSU a 300 watt PSU. After reading however it seems they underestimate the PSU it is more like a 400 watt. I know they call for 400 watts for the 5670 but it actually uses less juice than the 8600gts so I assume I would be fine there? My monitor is 22" and 720p. I don't want to spend anymore than $100 on a gpu.