With the advent of pci express and ddr2 memory I would deffinitly want to go woth the latest technologies. Tom's hardware has some reviews on some of the latest motherboards utilizing both of these. There are some problems with drivers,bios and board designs that need to be fleshed out. The Intel 925x Alderwood is the enthuisiast chip if you decide to go with intel. The Asus P5AD2 Premium is worthwhile of your consideration.
Ars Technica has a great article on pci express, I suggest you check that out to get an idea of the improvements this offers.
The western digital raptor SATA 74gb 10Krpm hard drives seem to be an enthusiast favorite. Personally I think I woul opt for a pair of 120 gb or 160 gb SATA seagates at 7200 rpm......cooler running.....quieter.
Ram at least 1gb of the best corsair you can get.
PS Antec true 550 decent amperage on the 12 volt rail plenty of power for all your upgrade needs.
Get custom round cabeling-color of your choosing to maximize cooling, fans gor for 120mm quiter and move alot of air.
Optical drives I like the Plextors of your favorite flavor.
Graphics cards....this is a biggy......the current pci-express cards are performing sub par to the AGP cards. This will change as drivers are tweaked to bring out the advantages to pcie. I believe the dual graphic card setup would easily fall in your price perimeters.
Of course all this is dependant on if you decide to go with Intel over AMD.......heh heh.
My bottom line suggestion is you wait a wee bit (won't be long ) for some of this to flesh out and be reading up on some of this. The PCIe is worth the wait in my opinion.......dual graphic cards could be a great way to go.
Glad to see you have decided to accept the challenge of designing your own system!!
nothing ventured.......nothing gained