Yes probably it's going to be badly bottlenecked. One 6970 is the max you can get for an FX-6100, You can however get one 6970 spend on FX-8150 and a good cooler (or maybe X4 980), overclock it and crossfire it when you have the money
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i think you may have misread his post, he said hd 6790 not 6970. i personally have 1 HIS hd 6790 with my athlon ii x2 250 and get decent performance. though, the stock fan on my card crapped out within 3 weeks, i have a left over 120mm case fan "rigged" on it til i get my GELID Solutions GC-VGA02-01 vga cooler. i would suggest a different brand (didn't bother trying to rma, i can't afford the down time ... sold my 9800gt and 9600gso i had sitting around). for xfiring i would recomend the hd6850 as it uses less power than the 6790. or get a 6790 now and a 6850/70 later to xfire. if you can get the 6790 for around $100 i would get it (that's why i got mine, i was going to get a 6770 but the 90 was the same price)