Well, you'll definitely need a new video card. The good news is your processor is just above the minimum requirements. The bad new is "just above the minimum requirements" means that while the game will play it won't play very well.
If you do want to go ahead with it your PSU is 300w. For an old 300w PSU the most I would recommend is probably an HD4650 which should be ok for low resolutions. The AGP version is overpriced though. There are 2 versions DDR2 or DDR3 with the second being better but often more expensive. This would be a good deal but it's out of stock;http://www.overclockers.co.uk/showproduct.php?prodid=GX-113-PC
Here's one that's in stock but DDR2http://www.ebuyer.com/product/179446