If one does not know how to build a desktop pc, you could use a site like
http://pcpartpicker.com to help you choose the parts that you need.
Gaming systems are more demanding than an office pc, so a good cpu and video card should be the priority.
A cpu at least 3.5GHz
A video card at least 2.Gb memory
4Gb of ram
This is a good place to start. More performance would require more funds.
I used to go to alienware.com and build some great machines, then I would go to TigerDirect or NewEgg to get the parts.
The last PC I built was for my step-son, and on the Alenware site, the PC was going to cost $6400.00. Yes it was a bit steep for a PC, but I did manage to build the baddest PC Alienware could come up with. When I did purchase the components from TigerDirect and NewEgg, the PC ended up costing $1500.00. That is still bit pricey, but he did have a desktop PC that was basically an Alienware computer, except it did not have the logo of Alienware. I did reuse some components to lower the price, like using the hard drive, monitor, keyboard and DVD/CD drive from his original desktop PC.
You could even search online for "budget gaming pc" and see what you come up with. Most of the time, they are already built, and usually only require a monitor, keyboard, and mouse to make them fully functional.
Hopefully I have helped you with your deciding thoughts of whether you should purchase the item you mentioned, or build a system yourself.
If you give me a price range, I could possibly come up with a decent system for you.