I'd definitely go the way of dual core, even if apps can't make use of it, it means that you can use the rest of your computer more fluidly while rendering.

You can get a lower end dual core and simply overclock it, remember that. (after market cooling)

As far as motherboards go, DFI is always a good brand.   Asrock is cheap if you'd rather go that route.

Memory - Go with 2 gigs or more if you're overclocking and want to run your ram 1:1 you're going to have to get a high speed ram (500 - 600). 

If you want to replace your hard drive there's a couple of routes you can go.   Raid 1 IDE or SATA isn't a bad idea if you get a motherboard with a good controller.  You can also go with raptors or even raided raptors, although the price difference makes the raid 1 Ide's or satas a much better deal.   (you can get 2x 200 gig drives for around $150 nowadays)

If you're not gaming a video card won't matter so long as it can render the open gl portions of your apps.   Obviously you could go with a fireGL if you want to waste money, but I'd recommend a PCIE ati x700 or something along those lines.

My setup is as follows, but I definitely wish I went with dual core:
Asrock Dual-sata 939 board
AMD Opteron @ 3 Ghz
2gb Redline XP4000
Radeon X850 XT-PE
2x 250G seagates

The whole thing is cooled by two watercooling loops, one pump / radiator for the northbridge and video card, and a second pump/rad for the processor.  Works well enough.