pci-e is faster than agp, it is the new standard for graphics slots.
You can buy a new mobo, sure, but you'd have to pay more $ and then
practically reassemble your pc. If you're going to do that then just build your own, it'll be cheaper and faster. Otherwise, if you can buy from Newegg, just pick up one of these.
ABS is a very good PC maker. This comp will give you a faster processor, a decent amount of RAM, better integrated grafix, and a pci-express slot if you wish to upgrade. Plus, it's not an Acer. Acer computers suck, trust me, I have to deal with them and their problems plenty on a daily basis @ work.