I think 5000+ BTU's should easily handle a room that size.
I tend not to like the quality of what I find at Walmart. Stuff bought there seems to die young. I like quality like a White Westinghouse A/C that my parents bought. It was manufactured in August 1986, and it still works. It's 5050BTU and draws 4.9 amps, which I think is about in line with the ratings I see on new air conditioners. Oddly enough, it's rated 7.0 amps.
I've had good experiences with Kenmore brand appliances. Sears has a 5300 BTU air conditioner for around $130, model 75051. I just bought it two days ago, and haven't installed it yet, but a 10,000 BTU model I bought over a year ago seems to be doing just fine, and it's pretty quiet, especially compared to the ancient behemoth it replaced.