wasn't sure if this has been talked about before but I wanted to tell you guys about 2DF Freeplay. its like GGPO only easier to use (for me anyways).
http://2dfighter.com/
just register and follow the directions. Its a lot of fun, plus there is very little lag. I love it. I usually play 3rd Strike, Twinklestar Sprites, SF2 or one of the shmups or beatemups on there.