gutharius, consider the Intel 875 + 82541GI CSA solution with some flavor of hardware RAID. For example, ABit IC7-G and dual 73GB Raptors in RAID 0. Get an 800MHz FSB P4 and dual channel memory kit, 512MB is fine. In theory, the new Intel 925 would be even better at this, but it sounds like that chipset has problems right now.
NForce3-250Gb claims to do the same thing on an A64, and there are claims of a NForce2 spin with the same capabilities. I'm really skeptical about NVidia and networking, though. At the very least, I would never want to be the early adopter of an NVidia product - give them 6+ months to bake the drivers.
Samba is pretty good, Linux is good, but the SMB protocol and the Windows client side is not so good. Keep that in mind when building.
For amusement, you might consider downloading Services for UNIX 3.5 and trying both NFSv3 and SMB between your clients and servers - if both will functionally do all you need, go with whatever solution gives you best performance. I believe that the Linux kernel-land NFSv3 server will give much better performance on that end, but then, Windows clients are optimized for SMB... so it's an interesting experiment for someone to go do.