I am currently running a Core i7 in my rig, and I HIGHLY recommend this processor! I was able to take the Core i7 920 from 2.67GHz to 3.8GHz on a $25US air CPU cooler! The processing power is absolutely amazing.

Motherboards for the Core i7, I believe, are pretty much the same across the board...they all utilize the X58 chipset (unless things have changed, I've been out of the hardware loop for a few months now since I built my current PC). My last two motherboards were manufactured by EVGA. While this company is fairly new to the MB building business, their products are great quality and very friendly to the novice or power-user.

I use my desktop mostly for gaming, and I have a variety of games that all place different demands on the hardware. For example, MS Flight Simulator X depends heavily on the CPU so my video just sits there yawning. On the other hand, Crysis crushes the GPU and is not very dependent on the CPU. Then some games, like GTA IV depend on BOTH the GPU and the CPU for top performance.

The only hardware that does not get fully utilized in my PC is the RAM...I currently have 6GB (remember, the Core i7 is triple channel, so RAM has to come in multiples of 3). Like kcarlile said, spend more on the GPU and CPU and not so much on the RAM.

I have NO experience with ATI cards at all, I've always been a nVIDIA fanboy so I can't help you with your graphics card. I currently run a EVGA Geforce GTX 275 FTW and couldn't be happier with it...it crushes every game I throw at it! I don't know what the ATI equivalent is, but a card like that should do you just fine!

If you are comfortable building a PC, then I would highly suggest you purchase your parts from Newegg (www.newegg.com). Great prices, great service. I was able to build my current PC for about $1200, all parts from Newegg (this was in July, so I'm sure prices have dropped a little since then). Here are the specs so you get an idea:

Core i7 920 OC'd to 3.8GHz
Cooler Master Hyper N520 CPU cooler
EVGA X58 3-way SLI motherboard
6GB OCZ DDR3-1600 Triple Channel RAM
EVGA Geforce GTX 275 FTW
2 Western Digital Black 1TB SATA-II HDD
XClio 850W GreatPower PSU
XClio A380 Full ATX Case

Hope this helps, please let me know if I can offer any more help or information!