Get a new PC. If you try upgrading the graphics you will need a new PSU and after that your CPU will be a bottle neck and the cycle continues.
People just love to spend other people's other people's money....
Seriously though, if your PC still does what you need it to do, then you don't need to upgrade.
As zenmaster pointed out, a fresh install of Windows and another gig of memory will definetly help with what you are doing.
If you wanted to start playing the modern more demanding games, it may be time to move on with a complete upgrade. I would not spend money on an AGP video card for a 4 year old system. If you are going to buy new memory and a new video card both, it might as well be DDR2 and a PCIe video card placed onto a new motherboard with a decent Core 2 Duo processor.