It's not necessary to have 3 hard drives. 2 is a good idea; 1 for linux/XP, and the second being a large drive for data. Maybe going with a 74 gig raptor for the linux/windows drive would be a great way to go, and then get a 200 or 250 gig data drive? If the 74 gig raptor is too much $$$, then the 36 GB will do fine too
Basically any motherboard, cpu, and gpu will work in linux just fine. ATI's 3d drivers aren't as good as nvidia's right now (you get 3d working fine, it's just quite a bit slower than windows).
I just did a system up at newegg for $900 that has the following:
A64 3000+
1 GB kingston value ram
GF 5700 ultra
200 gig WD SATA drive
36 gig raptor SATA drive
8x NEC dual layer DVD burner
MSI K8MM-ILSR mobo
You can use this as a base and then take whittle it down to get to $700 if you like. I figured the A64 would be the way to go because you can always recompile your kernel in linux so you have a 64 bit system (which will make the rendering and photo stuff much nicer).
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