We built two identical systems at work last week, the only difference was the video card. One had a Sapphire HD4850 and the other a Zotac 9800GT, the benchmarks were very close but the 4850 scored around 30marks higher on 3DM06 - not a lot but enough. I'd have to bank the 955BE for CPU too - more current and man those chips pack a punch. Keep in mind as well, 955's are based on the AM3 socket (AMD's current socket) where a Q6700 is skt775 - soon to be completely discontinued by Intel in favour of i5/i7 CPU sockets.
If you're going with the 955 though, make sure you get a DDR3 board or you'll only be holding the CPU back. I'd recommend any Gigabyte DDR3 board to suit.