I would say the i7 930, it's the chip I'm going with for my new build.
Newegg.com - Intel Core i7-930 Bloomfield 2.8GHz 4 x 256KB L2 Cache 8MB L3 Cache LGA 1366 130W Quad-Core Desktop Processor
This chip is Socket LGA 1366, so you'll need a Socket LGA 1366 motherboard. There are a lot of these around but you must balance price and features. I was looking at an ASUS board but then found this Gigabyte for cheaper. This board is ready for the future with USB3.0 ports and SATA3 ports for high speed devices. It also has PCI Express slots for new graphics cards.
Newegg.com - GIGABYTE GA-X58A-UD3R LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard
As for graphics you could go with cutting edge or an older model. ATi and nVidia (the two leaders in graphics cards) both have released fairly new products, the ATi Radeon HD 5000 series and the nVidia GeForce GTX 400 series. They are pretty close in performance and it comes down to price and features. I originally wanted a GTX470 but then after reading some other threads here convinced myself that the RadeonHD 5870 was a better choice, so that's what I'm going with.
I would build around the CPU. These days almost all graphics cards use the PCI Express interface, and all new motherboards (both Intel and AMD) support this interface. If you find a processor you like, then find a suitable motherboard and as long as it has a PCI Express x16 slot you can stick in a new graphics card (if it has more than one PCI Express x16 and supports CrossFire/SLI you can use multiple graphics cards together).