Off the shelf computers have good headline parts. core i7 processor, 6 GB of RAM, etc... , but they skimp on the case, PSU, use all 6 memory slots so there's no room for upgrade, etc... .
If you want to buy a pre made one, I suggest going to the dell outlet. You can buy returned PCs that are basically brand new for a big discount. Get on their mailing list and you'll receive 15% off codes several times a year.