I think there may be a very important part of information you're not providing and would likely help people give you better recommendations..... This is for AGP isn't it?
Anywhoo, either way, it is a very noticeable upgrade (as long as you have a good CPU), but I agree with Mac, you may need a new PSU. As good as Fortron PSUs are, that's alot of draw for a 400W aging PSU, you could try it, but I'd expect problems. At which point if you need a new PSU might it make sense to simply get a new mobo and PCIe HD5670 which would outperform both and likely still work on that old PSU.
A little more information about the system and pricing would be helpful.