Is this pre-built system a good buy?

true. the $$ savings of building it yourself aren't anything like they used to be. Back in the day you could save 1/3 or more on the price by doing it yourself. these days there is little savings... the main advantage to DIY is you can build the system you want to your desired specs. and the fun of a hobby project. if you can find a prebuilt pc, built to your needs, and you don't care about the hobby aspect... go for it.

as to the pc in question.

pretty poor use of your cash. if you're spending $900 on a computer you need to get a better processor then an old Llano, and a better gpu then a 7850.

at 900 i wouldn't settle for anything less then a 7870XT, or 7950... and i'd definately (if you're going amd) grab myself a Vishira... like a 6300 or 8320/8350.
 
Shockingly maybe. I tried to make a similar build. We don't know the PSU or exactly which GPU.
PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD FX-8120 3.1GHz 8-Core Processor ($154.99 @ NCIX US)
Motherboard: Gigabyte GA-78LMT-USB3 Micro ATX AM3+ Motherboard ($54.99 @ Newegg)
Memory: Kingston 8GB (2 x 4GB) DDR3-1333 Memory ($50.40 @ NCIX US)
Storage: Seagate Barracuda 500GB 3.5" 7200RPM Internal Hard Drive ($55.98 @ Outlet PC)
Video Card: Sapphire Radeon HD 7850 1GB Video Card ($154.99 @ Newegg)
Case: NZXT Phantom 410 (Red) ATX Mid Tower Case ($108.98 @ Newegg)
Power Supply: Corsair Builder 430W 80 PLUS Certified ATX12V Power Supply ($38.98 @ Amazon)
Optical Drive: LG GH24NS95 DVD/CD Writer ($17.98 @ Outlet PC)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.98 @ Outlet PC)
Keyboard: Logitech Wireless Combo MK260 Wireless Standard Keyboard w/Optical Mouse ($29.27 @ Amazon)
Keyboard: Logitech MK200 Wired Slim Keyboard w/Optical Mouse ($21.96 @ Amazon)
Total: $778.50
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-04-12 13:05 EDT-0400)

I'd still rather make my own. You can pick exactly what you want.