Well, your board has a single PCI-E slot so you can't crossfire. I'd say go with the 4870 1GB then. It will allow a greater range of games to play at that resolution. I also recommend getting the 1GB version as more RAM will help you out at those higher resolutions. Just don't expect to play Crysis on high at that resolution
. One of the things I like about CRTs is that setting them at a resolution lower than their max is usually fine, unlike LCDs. Even though it can go higher I play most of my games on my 19" NEC CRT at 1280x1024, but Crysis I run at 1024x768 at medium and it looks just fine.