What computer should I get to run Oblivion?

The computer I have right now is a 2003 HP desktop. It's got, I think, 180 gigs (hooray!) of hard drive space and an NVIDIA 3 series video card. I recently purchased a copy of the game Oblivion. It installed correctly, but when I went to play it, a small black window opened for about three seconds, and then closed, and the error message "Oblivion has encountered a problem and needs to close" showed up. I don't know what the problem is, exactly, but I know I need a new computer. What kind of computer should I get to run games with extreme graphics? Thanks!