Choosing a new graphics card can involve a difficult and lengthy hunt for comparative benchmarks to work out where to put your cash. Fortunately, Tom's Hardware Guide have put together a handy interactive comparison chart which can help reduce this problem to the simple act of staring at a few graphs.
It may be reasonably old, but it's useful, with a few caveats; not all of the cards were tested on identical systems, so some of the results are a little surprising -- clicking on the individual bars of the results reveals the relevant system specs. Also, although it is "updated on a regular basis", several cards are missing from the list. Still, if you're in the market for a new graphics card, this should help simplify your search.