Assuming you have a capable enough PSU. If you have a weak PSU though, then go with a Radeon 5670. Spending more money than that doesn't really make sense since you would generally be held back by your old CPU.
Yeah, as long as it's decent and has enough amps on the 12v rail. Something with at least dual 14A 12V rails or a single 20A 12V rail should be enough. If it's less than then you may need to go with the 5670 or get a better PSU