Only $10 more than the least expensive mobo, this major name brand offering has all the connectors you would need. It also overclocks and can handle two video cards well. A no-brainer for your configuration.
OS: Vista 64-bit with Free upgrade to Windows 7 $110 http://www.newegg.com/Product/Product.aspx?Item=N82E168...
If you'll never go beyond 4GB memory, 32-bit is fine. If you don't have any really old applicatons or hardware on your PC, and if you want to leave open the possibility to go to 8GB, you'll want the 64-bit version. Both cost the same.
This totals $646 in parts without looking for combo deals (will save $10-$20). We need a case and video card, so the total will be over budget.. You need to know this configuration is much more than you need and costs maybe $200 more than a PC to surf the web and run most apps you mention well. OTOH, depending on the vid card, there's *nothing* it won't run well . . . for quite some time to come.
I would consider a different mobo, just because that one is micro-atx, and doesn't really work too well with dual graphics cards (x4), it is a great board though. Also, I would definitely get a Spinpoint F3 500gb HDD for the fastest performance. If you decide on a ATX board, I would just go with an Antec 300. For a GPU, yeah, we do need to know your res, but check out the 5770 or 5850...
the price of amd quads is coming down and might help give a better value. and also help staying in budget. should have no problem staying in budget if your willing to compromise a bit on parts. "latest and greatest" and "budget" aren't usually used in the same sentence.
I would definitely suggest the Phenom II x2 or x3. This would definitely cut costs while still giving you great performance for what you plan on doing. By going this route you can put up a little more cash for the GPU. If you're dead set on using an Intel CPU you might have to increase your budget a little since you'll have to stick with the i5 or i7 (socket 775 is basically dead).
I would consider a different mobo, just because that one is micro-atx, and doesn't really work too well with dual graphics cards (x4), it is a great board though.
The GA-P55M-UD2 would be perfect for the OP's needs. He said he wasn't interested in SLI or crossfire. His budget also doesn't allow for a more expensive motherboard. The OP said he doesn't need an OS, so that adds $110 to the GPU budget. By the way, what OS are you planning on using?