$50-100: Find something on ebay/craigslist or save your money
$110-150: GTX 460 768MB (460 1GB on rebate)
$150-200: GTX 460 1GB or AMD 6850
$200-300: GTX 560 Ti or AMD 6950 2GB
$300-400: GTX 570 or AMD 6970 (only if your chicken to unlock)
$400+: GTX 580
Cards NOT TO GET
GTX 550 Ti, its shit its over priced and the 460 768MB is better and cheaper
AMD 6790, its shit and again its over priced as well as out classed by the 6850 for $10 more
Do I get AMD or Nvidia? Depends:
AM you get overall better power consumption in most cases as well as 3 monitors if your willing to spend the extra $35 on a Display Port adapter or on a monitor with Display Port.
Nvidia: better Direct Comput performance (on the fly texture compression like Civ 5), better Tessellation (major DX 11 feature), Linux support (some would say windows drivers too :P )
For my money I would go Nvidia over AMD for the Direct Compute and Tessellation since I think they will both be features highly utilized in games over the next two years. Thats said most people would rather have 3 monitors and the best overall feature set for the least money and thats why my Hardware Buying guide is AMD bias in the graphics department.
I never recomend SLI or CrossFire. If you are OK with sometimes only getting 1/2 the performance of your SLI/CrossFire setup, dont mind the extra cost on your motherboard, the extra expense of a 750w+ Power Supply, not to mention the added heat and cost of power, than your more than welcome to spend 2x as much money on two cards.