First off, considering you already have $150 saved up for this, my recommendation is to use patience ( its a virtue, I've been told) and get another $70 ( it includes tax) saved so that you can spring for this: http://www.amazon.com/EVGA-GeForce-GTX480-Graphics-015-P3-1480-KR/dp/B004EHWMP6/ref=sr_1_1?ie=UTF8&qid=1347863885&sr=8-1&keywords=480+gtx
that card is currently ranked #9 on the planet as opposed to this which is in your budget range: http://www.newegg.com/Product/Product.aspx?Item=N82E16814102908 the number 30 ranked card (admittedly I have this in the PC I am currently using, and it works great, no complaints but I can't run everything on the highest settings)
But as others have said, in order to ensure optimal performance for your card (whatever you choose) you will probably need a quad core, and you will almost certainly need a PSU upgrade....
my rankings are pulled from this chart: http://www.videocardbenchmark.net/gpu_list.php