Maybe I missed it, but I think you forgot to mention what kind of budget you were looking at, however, based on your choice on those bidget CPUs, I think I can work with that. May I ask why you want a 290/X? A GTX 970 outperforms it in most situations and does so with better thermal output as well as less overall power consumption. I will take a crack at this.
The AMDFX-6300 will simply not suffice, as I am sure there will be slight bottlenecking especially in older games that use only 2-4 cores. Not to mention, the chip gets hot, especially for a home theatre PC.
PCPartPicker part list: http://pcpartpicker.com/p/FM4Ht6
Price breakdown by merchant: http://pcpartpicker.com/p/FM4Ht6/by_merchant/
CPU: Intel Xeon E3-1241 V3 3.5GHz Quad-Core Processor
This CPU is priced similarly with the Intel Core i5-4690K, except it uses less power and has eight threads. In a small case, the thermals should be better as well as the power consumption. Keep in mind, it is not overclockable, but in a small HTPC chasis, I don't think you would want to.
Motherboard: ASRock H97M-ITX/AC Mini ITX LGA1150 Motherboard
This Motherboard is an inexpensive yet quality motherboard packed with several features such as built in WiFI while maintaining such small form factor.
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1866 Memory
Great kit of high speed memory. 8GB is plenty, especially if your main use is gaming. What's more to say?
Storage: Kingston HyperX Fury 120GB 2.5" Solid State Drive
This would be your boot drive and storage for large programs such as Adobe products that may take a while to launch. An SSD should have you booted in a matter of seconds.
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive
1TB of storage at 7200RPM. 'Nuff said.
Video Card: MSI Radeon R9 290 4GB TWIN FROZR Video Card
Nice card, but it generally runs hot. I would definitely switch it out for a GTX970. Will play all your games at relatively ultra high settings.
Case: Fractal Design Node 304 Mini ITX Tower Case
Great looking case as well as well built. Cheap and well worth the price.
Power Supply: SeaSonic 520W 80+ Bronze Certified Fully-Modular ATX Power Supply
SeaSonic makes great power supplies, and 520 watts will be plenty to have your system running stable.
Don't forget to grab a copy of Windows 8.1!
Personally, if it were me, I would go with this build but swap the 290 for the aforementioned GTX970. The Intel CPU will do wonders, but if you would like me to make a cheaper solution, I can get back to you in the morning as it is quite late.
Cheers,
-Abhorrence
PS: Not sure if I mentioned, but the architecture in the AMD FX-6300 is 2011 technology, whereas the Intel Xeon CPU is still fairly new.
If you want something cheaper, be sure and let me now and I can try my best to help you out. You didn't really give me a budget..