I don't necessarily agree with that, but for right now, unless you want to completely revamp your entire system (new graphics, RAM, CPU, motherboard) you might want to keep AMD for a while.
This motherboard has AGP, DDR, DDR2, and Core 2 Duo support, but I've never heard of the manufacturer:
http://www.newegg.com/Product/Product.asp?Item=N82E16813185086
Note that it's MicroATX though.
I would just upgrade to something like an X2 4800+ for now:
http://www.newegg.com/Product/Product.asp?Item=N82E16819103544
Also, you might want to upgrade to a PCI Express video card, this card is the best price vs. performance value:
http://www.newegg.com/Product/Product.asp?Item=N82E16814130062