1. The one thing I'm really unsure about is the motherboard. I'm thinking that in the future I may want to get another 4870 and Crossfire them, and I know X48 would be the way to go, but just unsure if this motherboard is the one I should get. Also, I would like to try overclocking the CPU, RAM, maybe video card, and not sure what the best mobo for that is, either. And if Crossfire in the future is a bad idea, is there a P45 board I should look into?
2. For the RAM, I just chose 4GB of DDR 800 that had the lowest timings at standard voltage (5-4-4-12 at 1.8V). Is there a different brand that would overclock better, and possibly give me better performance overall?
3. For this computer, loudness isn't an issue (notice the Antec has six fans). I play games with earbuds, and can't hear my computer at all. I also want to get the computer as cool as possible. Are there any case fans I should be getting that will really help with cooling?
I think that's all my current concerns. If you can help, I'd be really thankful.
Good motherboard you pick. I suggest staying with it. Your RAM is good, make sure it's on the motherboards list of compatible RAM first to be sure. I honestly can't tolerate loud computers, and don't recommend buying more fan than the case comes with. If it comes with intake fans as well as outtake, that's plenty.
No idea about that tuner. I have an older Hauppauge model (PVR-150) for regular TV and it's great.
The rest of the build is excellent.
Yes, the GA-X48-DS4 is a good choice. It can bring the E8400 to 4.2 GHz for example. It's also perfect for HD 4870CF. You should consider CF with that monitor, it's a good idea. Your PSU and MB and case are fine for it.
Fans: SFF21F sounds best for you. 30dB/1600rpm.
Good RAM. There may be better choices, I don't know, but the one you picked is good and has a very nice price.