The best I can get for your $800 budget.
CPU-Intel Core i5-4460 3.2GHz Quad-Core Processor
Motherboard-MSI H97 PC MATE ATX LGA1150 Motherboard
Memory-Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory
HDD-Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive
GPU-MSI GeForce GTX 770 2GB TWIN FROZR Video Card
Case-NZXT S340 (Black) ATX Mid Tower Case
PSU-EVGA 600B 600W 80+ Bronze Certified ATX Power Supply
Total: $799.93
My personal choice (if you change your budget)
CPU-Intel Core i5-4690K 3.5GHz Quad-Core Processor
RAM-G.Skill Sniper 8GB (2 x 4GB) DDR3-1600 Memory
GPU-Gigabyte GeForce GTX 970 4GB
Motherboard-Gigabyte GA-Z97M-DS3H Micro ATX LGA1150
HDD-Western Digital Caviar Blue 1TB
Case-Cooler Master HAF 912 ATX Mid Tower Case
DVD Drive- Black SATA 24X DVD Burner
PSU-EVGA 600W
Total: $878.69
Both are NOT pre-built as you requested, I don't know where your from and therefore do not know your local Hardware stores etc. Sorry
I would suggest buying a PC from a place where you can pick the parts yourself and get them to assemble it for you, its generally a lot cheaper!
Edit: Use
http://pc.ncix.com/pcbuilder/ to build the PC for you, I think there's a charge for it but it's worth it if you want to avoid the hassle of building it yourself!
