Should I upgrade my computer or buy a new one(see details)?

go with the new one, AGP is redundant, PCIe is in.

upgrade

if u have an AGP slot u no u need a new comp

If you have the money, you should buy a new computer. If not get the video card, it'll hold you over until you get the money to buy a new computer. If it were me, I would get a new computer.

If you plan on playing games beyond WoW, just replace. The Athlon 2600+ is really obsolete, too, and hearkens to 2003 or so. Nothing but a new motherboard and CPU will make this machine modern, so you'll need to buy something else for more serious use. Look for something cheaper, maybe.. anything with a pci-e slot should be okay.

What are you doing with it? It is only if you are a gamer that you will presumably want a better graphics card. The best cards these days are all PCI Express which your machine can't handle, but there are good AGP cards out there still. Since upgrading to PCI Express would be a major upgrade (at least a new mobo and new graphics card, and likely new memory and CPU too) there isn't much cost advantage over a new PC where of course you still have the original machine for something else. I would be tempted to drop in a new graphics card on the understanding that the machine as a whole is reaching the end of its life: in other words it is only putting off the day it needs replacing a year or two. If you are not a gamer, I would look at RAM and possibly the disk controller - you never get good throughput from onboard types and need something much better specced for best results. I'd check CPU utilization but I doubt that that is much to worry about.