How much money do you have set aside? That will ultimately determine what kind of parts you should get.
For CPUs, Core i7s are indeed pretty powerful (although I'm running a C2D E7400 just fine). If you don't have ~$300 for a CPU, you might consider the newer Core i5 line, which is a budget dual-core version. There's an even cheaper Core i3, although I don't know much about it.
For GPUs, I'm told that ATI's Radeon HD5xxx series is better than most anything offered by Nvidia. Really, though, I don't think the difference is that huge, so either brand would be fine. Make sure you have at least 512MB of video memory, probably 1GB if you'll be running high end games. I'm sure a lot of other people around here have more specific suggestions.
The motherboard you get will be dependent on what kind of CPU you want. I really don't know what brand is best in this case. Things to consider, though, include how many SATA and PCI-e ports there are, how many slots are available for RAM, and the fastest memory supported.
I think you'll want to spend more of your time thinking about the hard drive. This is typically the slowest part of your system. If you have a lot of money to blow, consider the
Intel X-25M solid state drive - they are considerably faster than conventional drives. Obviously, they're much more expensive, and they don't store a lot of space yet. For conventional drives, Seagate/Western Digital are considered the most reliable brands. I'd get something from them with at least a 7200RPM spin rate and 32MB of cache - watch out for the 'green' lines, as they're typically slower.
Hope that helps a little!