Here is a tip I think might help. I know this isnt for gaming, but 1 way to tell if a generic prebuilt machine is of any quality is the type of PSU.
If they care about their quality/reputation they wont put in some chinese-mase generic POS PSU.
Thats my theory.
Or if the say only a "500W PSU", you know it's going to be crap.
Both Very Good Points.