OK, FYI here's an up to date list of
graphics cards on the market, organised as a hierarchy from most powerful to least.
It's taken from
www.tomshardware.com, a site I highly recommend if you want to learn all about PCs and their innards.
This chart, for example, is found at the end of their monthly "Best Graphics Cards for the Money". If you go to the site you'll find the article on the front page. As I've said, it's a regular monthly article.
I use Tom's whenver i update my PC so I can see what's doing in the business. RAM, HDD, CPU, GPU, Motherboard: they're all covered.
I put an HD 5970 in mine. Overpriced and overkill at the moment, but I like to "future-proof" to last a few years if I can.
Lastly, it's important to understand 'bottlenecks', where your GPU, CPU or RAM becomes the 'weakest link' of the interaction of the three. For example, if your CPU is the limiting factor, shoving a more powerful GPU won't give you much of a return, if any at all. Tom's has an article somewhere on this, and they regularly look at what combines well with what. I think it's a great site if you want to spend the time doing some reading.
Cheers