Yes it HAS to be PCI-E.
It would be useless buying AGP now.
I can understand buying new AGP cards if you have an AGP board, but Im sorry buying an AGP board now is not a good idea
Get a socket 939 mobo
1GB pc3200 ram.
3000+ or 3200+
6600 GT or 6800 GT
I know thats probably over your budget but that should do nicley
BF2 will chew up and spit out that ram you have.