I got a massive spec dell (6Gb DDR3, i5 sandybridge, 750gb hdd, 1gb GPU) off dell outlet for under £400
I'd avoid packard bell and HP(terrible last few years, and crap after service)
Lenovo rock solid, Dell are good, as are Asus and to a lesser extent, Acer.
This is a general view, everyone will have had issues with certain brands