3dMark is a synthetic, IE mostly irrelevant for comparing different cards. The difference in 3dMark isn't quite the difference in real-world performance. Still, you may have been a little overly optimistic about the performance improvement.
If you're upgrading your motherboard, then you'd probably be better off getting either an Intel setup with a dirt cheap board and multiplier-locked i5 or a decent AM3+ motherboard and an FX-6300. I'd recommend the former.
I'm not aware of any AM3+ board that doesn't support FX,although some may be limited to FX-x1xx/x2xx (Bulldozer) models and not support the FX-x3xx (Piledriver) models. However, I'd expect this to be fixed with a BIOS update on most such boards.