imo radeon hd 6850 is better than the radeon hd 7770. 7770 is currently quite overpriced for it's performance.
fx 6100 will be okay with a single 6850 as long as you don't do crossfirex.
it might bottleneck then. if you can find a deal on 6870, even better. i don't know your gfx card budget.
make sure you have a sturdy psu to support the overclocked fx6100. it will use up quite a lot of watts when overclocked.
i agree with esrever, an ssd will be good for fraps recording and gaming. may eb a crucial m4 or something similar, if your budget allows.
memory bus width is an important factor in measuring memory bandwidth. 256 bit cards, that is why, will always be better than 128 bit cards. when memory bandwidth is similar, other factors like core clock speed, memory clockspeed, underlying architecture are also other deciding factors.
lastly, don't believe right away if someone says that they can't see any problem. doesn't mean it's not there or it's not being mitigated by other factors or no one else should not or cannot notice the problems. that's why benchmarks from tom's are a good measure of performance.