Server motherboard/CPU upgrade advice

Thought I'd tap into the vast knowledge of TG readers to get some insight on a server motherboard and CPU.

We run a custom computer shop from home. We have an 8 port switch which is usually full and connects to a server running Windows 2003 SBS. The server then connects to a ADSL2 modem. We also have a 4 port wireless router which usually has a couple open slots, but that connects directly to the modem instead of going through the server. So basically we're talking about 8 machines every day 24/7.

2003 SBS is a must. We rely on Exchange daily and I'm quite happy with the security the OS has provided thus far. We can also monitor/control what our troublesome teen does on the internet. So the OS stays as is.

The server has been running an Asus P5WD2-E Premium motherboard and Pent D 3.0 CPU. It's due for an upgrade. This board/CPU has served us pretty well but I'm wondering if there's any real advantage to going with a server motherboard over a desktop board for our small network. I'm pretty sure we won't be going with dual CPUs as I can't justify the cost. I definitely think a Core 2 Quad is in order, and am leaning toward a Q6600 that I'd like to overclock a bit (3.0-3.2GHz). Also hoping to stay with DDR2 memory as we have 2 gigs in it now. I'm currently running folding at home on the server and would like to keep that if possible. We also run a file sharing/chat server on occasion.

Currently we backup half our systems on HDs on the server, and the other half via E-SATA. I'm much more impressed with the speed of E-SATA than the server, but E-SATA through the server would be a viable option as well.

How much hardware is necessary without overkill? Budget is a concern as we tend to upgrade all our systems every 1-2 years. Thoughts, opinions, criticisms? All input is appreciated.