DIY for SURE, you will save a lot of money, if you were to buy the equivalent PC for what you have in it from retailers, it will cost you significantly more.
I would suggest an ASUS,MSI, or Gigabyte board for your PC, I was looking in to ASUS boards for a good while and everything I have seen, read, and heard about them was good, MSI sometimes even better, they have some unique utilities and features compared to some other mobos too, I have a gigabyte, and I love it, it stays pretty cool, even with a miniature sun of a CPU with a not so good heatsink.
CPU I would go AMD, they are more cost friendly and give you more for your money compared to Intel IMHO. I got a Phenom II 940 BE quad core for half the price of the i7 920, and the only difference I see online is a few seconds processing time with video editing software, it shouldn't give you much of an advantage over games compared to an AMD, so if you're not a big media/ video editor then you will be more than happy with AMD.
Graphics I can't really give you a good idea on, ATI seems to have a few better technologies, but there's a few nvidia is working on that look pretty sweet, temperature-wise I THINK, but don't quote me on this, but I think that ATI has a small advantage on their upper level cards with the stock coolers (HD4800 series and up). Either way you go I think you would be happy though.