Is this a good computer?

Shopping for a gaming PC is incredibly simple. Go to youtube and search for the game you want to play and the specs you're considering. If you don't like the performance, try a more expensive part. Keep doing this until the performance/price ratio is something you like.

Sad truth is between youtube and google all of life's mysteries can be solved.

When I built my machine I watched a mess load of youtube vidoes seeing what people where getting out of the gpu performance wise.

The HDD is the biggest concern I have with this build, only 128 GB. Inside of games a SSD isn't going to be as much of an improvement as most people think. By the time you install windows you'll realistically have like 80-90 GB of data left on the disc and that's going to get eatten up really quickly by games that are 8+ GB a piece. Never mind music, pictures, and other media.  Where SSDs shine is outside of games they'll help with transferring data quickly, computer load times, and so forth. So the type of drive and the size is really dependant on what all you need the computer to do.

GPU is the 2nd thing I'd look at, I actually have the GTX 750 on my machine. It's a solid performing budget card that's amazed me on quiet a few ocassions. (It runs all the games I've thrown towards it on high @ 1920x1080, and I've seen youtube videos where it's played all the latest multi-platform games at better quality and frame rates than the PS4 & Xbox One.)...but let's be real it's going to be the first thing to become outdated on the machine as it's already on the lower end of the spectrum. So if money permits I'd do some research and see where above the 750 you can afford to go and be happy.