On first site:
Change the motherboard, it is meant for SFF cases, and is a very low end part - it even has onboard graphics. I recommend the Asus A8N-SLI Premium to go with the other parts.
Do you really need a 7900GTX? It would save you money if you went for a 7900GT (make sure its that, and not a 7600GT or a 7800 series card), and you'd only find a small drop in performance.
For the heatsink fan, I would recommend the Arctic Cooling Freezer 64 Pro, or the Gigabyte G-Power Pro over that one any day. BTW, make sure your super confident, because when you fit the 3rd party cooler you void the warranty. Although, it is pretty much impossible for the supplier to tell that you have been using a 3rd party cooler, unless you crush the core or something which is near impossible.
What about a monitor, keyboard, speakers and a mouse?#
Ah, I now see you are buying a MicroATX case. OK, I would recommend against it, or downgrade. The cooling won't be very good, and you'll struggle to get the 7900GTX in - I very much doubt it will fit, as it is very long and takes up 2 slots. I would recommend getting a normal case, such as the Akasa Eclipse or the Silverstone Temjin 5.
To build a Media Center PC, you should be looking at a Pentium M, which not only would give you excellent performance but is extremely fast at gaming too. In fact, I would recommend one of the new Core Duos, which would do you perfectly.