I would start by investigating if you will actually utilize OpenCL. If so, yes the HD7000 series is the way to go.
MMO's can be very demanding. You may wish to get an HD7970. It's up to you. WOW and Diablo 3 aren't very demanding but games such as Guild Wars 2 will basically take anything you can throw at them.
I'm not a fan of Crossfire due to micro-stutter. I don't want to start a huge discussion here but it is a big issue. In fact the new GTX 690 specifically includes a chip designed to reduce micro-stutter (dual-GPU) and NVidia admitted in an interview that micro-stutter exists, has been improved with the GTX 600 series but is still an issue.
CPU:
I recommend the i5-3570K. You can decide with benchmarks if the more expensive CPU is worth it. It won't matter for games so just look at transcoding benchmarks.
Motherboard:
Definitely a z77 system. Asrock is good. No issues there.
RAM:
It depends on if you are editing or simply converting/transcoding video. For editing, it's possible that more than 16GB would benefit you. I simply don't know enough about your requirements. If you think 32GB would benefit you then you'll need to ensure your motherboard supports it AND you'll need Windows 7 64-bit PRO, not just Premium as Premium only supports 16GB (I believe this is correct but verify yourself).
Compute/OpenCL:
It isn't widely supported yet. Things are happening but they can take years.
NVidia GTX 670:
OpenCL support is substandard. However, if you deem OpenCL isn't a big deal for this current card (can upgrade in the future) then I'm inclined to recommend the GTX 670 DirectCU II from Asus:
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670_Direct_Cu_II/33.html
Summery:
- i5-3570K best value CPU IMO
- get a motherboard AND case that support front USB3 (Antec for example has some inexpensive cases with cable managment, front USB3 and two fans)
- graphics card is the hardest choice. Recommend up to an HD7970 if OpenCL is important or GTX670 if it isn't.
- Z77 definitely. Asrock is good. Recommend Virtu MVP feature. It's a big issue but you do NOT need to use it. Too much to discuss here.
- CPU HSF: I agree on the 212 EVO. good value.
- RAM: 16GB probably. 32GB only if editing video and it seems necessary. Need the motherboard to support it AND Windows 7 64-bit PRO.
What RAM? I like G.Skill. The best value is probably 1600MHz. You may wish to investigate ECO ram (lower than 1.5V but it's absolutely critical that the motherboard support it.) G. Skill has 32GB kits of 4x8GB as well but that really doesn't matter as I doubt you save much money.
Other:
- run Memtest upon build complete
- buy the OEM version of Windows 7 64-bit
- 32GB versus 16GB adds $100. Power isn't too big a deal. You can always buy 16GB and upgrade but I don't recommend mixing so investigate this now and buy the amount you think you'll need. If it is needed the time savings can be huge. enough of that.
- front fan for case (i.e. 12cm 500 to 800RPM, 16dB, non-variable, molex or adapter)
- 120GB SSD for Windows (requires some research for quality)
- 2TB WD Green or similar hard drive (downloads, games if space demands, backups etc)