You might want to pick a single game, or a few games that look really fun, and that you're interested in trying out. Ask around on the message boards what you would need to run those games smoothly. Chances are, if they are newer, somewhat graphically-intense games (lots of fancy, realistic-looking graphics), and your computer can handle those, it can most likely handle most anything else on the market.
There is a sweet spot in computer building to get the best bang/buck. It's not necessary to buy the absolute best video card available, for example. This is assuming money is an object for you to some extent. Especially as someone just trying to get their feet wet with PC gaming, you should look for good values, so you don't spend an arm and a leg, and then figure out PC gaming is not for you. How anyone could come to this conclusion is beyond me though =P