go w/ an A64-2800 solution. Marginally more money than an AXP, (like maybe $80) and it will be more upgradeable. There are some decent A64 boards with video onboard too, if cost is a big issue, though if they are considering going w/ a 3700, I'm assuming cost doesn't matter that much.
What is the computer going to be used for? Go for a nice A64 (above 3200) if they are using it for CAD, video editing, 3d animation or something, otherwise it doesn't matter at all.
Cheap solution:
DFI Km400-MLV
Athlon XP 2500,2600, or whatever
512 megs pc3200 ram
80 gig hard drive (SATA or ATA)
Medium solution:
DFI K8M800-MLV
Athlon 64-2800 or Sempron 3100
512 megs pc3200
80gig SATA
Video editing and what not rig:
ASUS K8V SE DELUXE or similar (I'd stick w/ s754, unless money goes on trees, then go s939 and a really nigh end CPU)
A64-3200
1 gig PC3200
120 gig+ SATA, or if you really want to spend money go with raptors (don't bother with raid, not worth the $)
If stability is of the utmost importance:
Asus SK8V s940
Opteron 144 or 146
1 gig registered ECC pc3200 ram
I would be surprised if you ever get something w/ registered ecc to freeze up (for hardware reasons). You'll pay for it, but the stability will be unmatched.