Anytime you talk video cards, there are certain specifics you have to share when asking "whats good". First is how the card plugs in - PCIe or AGP - though your only going to see AGP in older motherboards (aka existing machines). Second is the power supply your going to use. Often people miss the mark on power and have all kinds of problems. Third is use - obviously gaming for most here - but are you wanting to push the envelope on the newest games? If so - share the CPU speed and memory you have in your box, as well as the planned OS. The reason for this is simple - if your running XP and not planning on moving to Vista (or 7 "when" its here) then spending extra bucks for dx10 compatibility is a waste unless you also get more performance in dx9. If your running a bottleneck at the cpu or system memory - the most expensive card isnt going to help you alot. Lastly, you have to have a price idea that you want to hit.
With all that said, I have to refer you to toms hardware.
http://www.tomshardware.com/reviews/...Card,2033.html
Every month they do a roundup of cards in various price categories by both Nvidia and ATI.
Take what you know about your machine - the need you have - and the money you have to spend - and see where the list points you. You wont be unhappy with the results if you put in the time and effort.