If you might use 3 monitors, then AMD has the advantage. If you might use applications that take advantage of uploading some processing from the CPU ( only a few do now but I expect a lot more to later) than nVidia has the advantage. In the majority of cases, the choice is not between the two as such but in looking at the performance of particular cards for particular games.
I recommend this deal - an EVGA GTX 460 768MB - a $190 card on sale and after rebate for $110 with free shipping:http://www.newegg.com/Product/Product.aspx?Item=N82E16814130564&cm_re=gtx_460_768mb-_-14-130-564-_-Product
The 768MB version performs almost as well as the 1 GB version:http://www.tomshardware.com/charts/2011-gaming-graphics-charts/compare,2676.html?prod%5B4830%5D=on&prod%5B4831%5D=on
And the 768MB version is on the current THG Best Cards for the Money list at the $150 price point:http://www.tomshardware.com/reviews/best-gaming-graphics-card-geforce-gtx-590-radeon-hd-6990,2879-3.html
But make sure you check your PSU and confirm that it has enough power. If you have questions about checking power supplies, you might review the relevant page at upgradevideocards.com:http://www.upgradevideocards.com/criteria.html
See the information on PSUs about 2/3 way down the page.