Those games are favored by NV cards but the 7850 is good enough and i think because of your PSU it's your best option the 7850 performance is in between a 560 ti and a 570 it's a bit faster then a 560 ti and a bit slower then a 570 i don't think your taking a big loss as i mentioned 7850 can overclock beast
MC primarily relies on the CPU. I never get over 25% GPU usage on my card, and even if I take it out I still get quite good frames.
If you are an avid player of Borderlands 2, as much as I hate to admit it, you might even should consider a 660 for the PhysX. I'm not a person who thinks PhysX is a buying point, but it does look nice no doubt. 660ti trades blows with a 7950, but the overclocking headroom is very nice. Although if you do get an AMD card you can still use a nvidia card as a PhysX card using hacked drivers. PM me and I'll give you a link on how to do it if you do choose to match an AMD master card with a Nvidia PhysX card.