Here's my opinion.
If money is critical, and something like the 6670, or 7750 is the limit of your budget, then you've already answered your own question. Go AMD, or Nvidia, the best card you can get for that money is mediocre at best.
When it comes to graphics cards, they age so quickly, and become obsolete in such a short period of time, that you need to purchase the very best card you can, within the constraints of the budget you have. It's the only way you can be assured of getting the most life possible from your new card.
You may want to consider this...
Newegg.com - EVGA 01G-P3-1556-KR GeForce GTX 550 Ti (Fermi) FPB 1GB 192-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
At least you'll be able to enjoy PhysX when available, excellent Tesselation, and very matured, and honed drivers.
If you can see your way clear to spend a little more money, you'll get a much better bang for your buck with a Newegg.com - EVGA 01G-P3-1561-KR GeForce GTX 560 Ti FPB (Fermi) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
What ever you choose, spend the most you can afford to avoid having to purchase another card too soon.
HAF 932 Advanced / Corsair AX1200 / Asus Rampage IV Extreme / i7-3930K / H100 Cooling 4x120mm
16GB Corsair DDR3 / GTX 580x2 SLI + GTX 460 PhysX / SSDs Mushkin 240GB, Crucial 256GB, Crucial 512GB