Well with a gaming PC, it's going to require alot of processing power, a good motherboard, fast memory, and a good video card.
For your HTPC( Home Theater PC), you're going to need a large hard drive for all of your media files.
But with high performance gaming components, they're going to give off alot of heat, and alot of noise from fans.
With a HTPC, you want to build a computer that gives off minimal heat, low noise, and has a quiet power supply that saves energy. So you're probably going to have to compromise a bit.
If you would like any of us to configure a build for you, don't be afraid to ask.