Do you want to spend a lot or just enough on a graphics card? I personally wouldn't spend more than around $300 for a graphics card. What are your other PC specs, Processor, and power supply?
Right now I have an AMD 7950 3GB and its doing me pretty good with shaders and 4x AA. Here are my suggestions for cards, in order of performance and price from low to high
HIS AMD 7770 $135
SAPPHIRE AMD 7950 $300
GIGABYTE GTX 670 $400 (Comes with Borderlands 2 for free)
Again I wouldn't go higher than the AMD 7950 but if you play other games besides Blockland or are just a hardcore gamer then go ahead an spend a bit more.