Is this a decent gaming computer?



It is good for the price but....
if you want to upgrade components later you should go with an intel system. Although they may be more expensive the best you can get with intel is a huge amount better than an fx-4100. But if you want to get the best cpu later on, you should get an intel one (e.g i7-3770k) and if you get an intel one you will have to get a new motherboard corresponding to the correct intel socket.
But overall this is a good system for your needs. And with the GPU side you could play at around medium/high(on some low-end games) and get 60fps
P.S Look at the benchmarks for the cpu and gpu. Just search "fx-4100 benchmarks" and "radeon hd 7750 benchmark"
Would you be willing to build your own system? This will play most games at medium-ish (some lower) settings and probably get you 60fps in minecraft with no mods on. The GPU is the weakest of the new generation cards and will only keep up for a short time before it will struggle with new games.