No, you definetly need a new mobo (motherboard), a new processor (proc), and probably... a new video card. Like... I have a POS Dell Dimension C521. And when I pop The Elder Scrolls IV: Oblivion in, the game Auto-Detects my settings as High. And the game is beautiful, with chopping in intense fights, ie. getting hit by an enemy flare.
Though mind you, GRAW is a pretty intense game, system req's-wise. But, if you're still using AGP, then don't expect much out of newer generations of games. You can play... Hmmm... FarCry on High... though thats no GRAW.
I suggest going with a newer ASUS or MSI board, and pick between AMD or Intel (though right now Intel definetly has the upper hand) for your Proc, and go PCI-E with your Vid Card.