If this is primarily a gaming rig, I'd go with the Barton and spend the difference on the rest of your system (vid card, sound card, ram, etc.) The 64-bit systems are really nice if you run 64-bit apps, or you use multiple 32-bit apps that use lots of memory, but for gaming, they're a waste of money. I definately wouldn't go with the Opteron, you'll get faster game performance from the Barton. If game companies start releasing 64-bit opimized versions of their games, this situation changes, but I'm not holding my breath on that one.
The 9800pro is and extremely nice card. While I typically use nvidia hardware for workstation level use, my game rig has a 9800 in it and it performs very well.