I'm currently in the market for a new desktop for some light to medium gaming (MMO's and strategy games) and general internet/office use. My first thought was to build my own, but then I encountered some good looking deals in refurbished computers.
My budget is around $400-500 including OS and peripherals (except monitor).
The only reason why I'd go for a pre-built system is because the manufacturer (assumedly) took the trouble of making sure the patches match up properly and work well together. I'm not sure if you can pick a good GPU for that microcenter system, and you might have to replace the PSU to accommodate the new hardware.
In short, go for the homebuilt system. Good luck, and don't forget to use this format when asking for build advice!