I got the cheapest Dell 14" (Core i3 w/4GB of RAM). I think I paid about $400 shipped. Works fine for me. I've had it about 2.5 years, no issues at all.
I run Windows 8 (going to revert back to 7 soon) and Ubuntu (which I use unless I need a Windows program). No issues there either.
I kept watching Slickdeals until they had a sale:
http://slickdeals.net/I think some makers are starting to offer computers with Ubuntu which might save you a few bucks too. Personally I like Ubuntu much better. The only downside I've found is you can't watch Netflix.
For brands I like, Dell, Lenovo, and Asus the best.
Pretty much anything with a Core i3 and 4GB of RAM should be more than enough, and be a half decent quality. When they start to cut corners with AMD processors I think the build quality of the entire thing usually comes down with it.