The Rampage Formula is one of the best from what I've read. I have it's brother, the Maximus Formula. Great board though the X38 chipset kinda sucks with 45nm quads. X48 is supposed to be better though.
I'd avoid Intel motherboard since they usually lack the features other boards have.
Gigabyte is good too. I believe they have some high-end X48 boards as well. Another option is DFI though the BIOS settings can be a bit difficult.
I find DDR2 to be a better value, but if you can afford decent DDR3 then go for it. However slow/high latency DDR3 is going to be worse than lower latency DDR2.
I would go with the Rampage...very popular board, and most seem to like it. The NB runs a little hot though but by all accounts its a great high end board.
I have the Antec Nine Hundred and love it. However, there is no filter on the front intake fans, so you will get a lot of dust in there and you will have to clean it up fairly often. You could try the Antec Twelve Hundred...its s full tower, has three fans in the front and two in the back, and has filters in the front to help keep some dust out. It will run about $50-$70 more than the Nine Hundred though, but at your budget you could probably fit it in if you choose.
Thanks again for all of your advice! I've revised my build based on your suggestions so hopefully this one looks a little better. It's slightly more expensive coming in at around $2,891.33 but I think it should be a good system overall. I tried an alternative system with a gigabyte DDR3 motherboard and DDR3 ram etc and it came in at $3,130.3. I can't bring myself to pay more than $3,000 for a computer that will be extinct in 6 months even if it isn't much more than the one I will list. I'm still debating changing the case up to an antec 1200 based on one of the filtration recommendations. Don't know if I need a 1000w psu but in case I ever buy anohter 4870 x2 down the road it would be nice to have. The only other question I had is if anybody has any good hsf to recommend for the CPU as I doubt I will keep the stock unless it's good enough. Let me know what you think of this and any more suggestions or alternatives would be much appreciated! Thanks!
Case: Antec 900 - $92.22 (Seems like a decent case) Mobo: ASUS Rampage Formula - $319.68 (Seems to be the popular pick right now) CPU: Intel Core 2 Quad Q9450 - $319.99 (CPU bottlenecking with videocard?) RAM: Mushkin Ascent 4GB - $139.99 Video Card: HIS Radeon HD 4870 X2 - $609.99 Hard Drive: Seagate Barracuda 1TB - $138.99 (Seems cheap enough for 1tb) Optical Drive: Samsung SH-S223F - $28.89 (Don't know much about this...) Sound Card: Asus Xonar D2X 7.1 - $182.52 (Kind of pricey but good quality?) PSU: Corsair HX1000W - $254.99 Monitor: Samsung Syncmaster 245B 24” - $419.99 Mouse: Microsoft Habu – $64.87 Keyboard: Microsoft Reclusa - $51.37 OS: Vista Home Premium 64-bit - $123.88 (32bit or 64bit? Compatability issues?) Speakers: Logitech G51 5.1 - $144.56 (not as good as z5500 but good enough?)