There's also a few things about that rig that make me think "overclocking attempt machine" - ie fan controllers, aftermarket cooler, huge wattage psu, the specific cpu used in the build. If its important to you, I'd investigate if it was used for that, as overclocking would have voided any manufacturer warranties (whether the manufacturers would know or not is a different beast). And theoretically if it was overclocked the parts could be damaged. So something to consider, don't know how close the coworker is and if they'd ever pull something like that. If it was used for overclocking its not necessarily bad, its just an added risk since technically it would have no warranty and if you don't know much about computers it would be difficult to know if anything was damaged in an overclocking attempt (unlikely but never know.)
Parts look fine for a mid range gaming rig, the PSU's a bit "no name" with a higher rate of failure than some, but probably as good as the generic you'd get in a dell or something. Ditto on the motherboard, biostar is pretty generic as you can get. The 560 is probably the nicest part, and the 16 gigs of RAM, though that's a bit of overkill.
As the previous poster said, see if it has a real windows OS disk with it; if it doesn't, buying one to get legal would be another 100 bucks at least for windows 7.
In all if the windows is legit, and setting aside other concerns, I'd say its a pretty decent deal, especially if you want a mid to mid high gaming computer. A 560 should handle a lot of current games on high settings, and the CPU shouldn't bottleneck it too much. You could probably grab a dell or something on sale that had a better CPU/ hard drive and such for about that same price, but you'd probably have to toss in 100-200 extra for a good gaming GPU.
