I'm going to be ordering a server to run Exchange 2007 for a 25 user network, Dell offers dual and single core processors as well as multiples of each. Is there any advantage in running quad core and/or multiple processors with Exchange? We have 3 remote offices that connect via Citrix if that makes any difference.

Windows and Exchange will certainly be able to use multiple cores, but a 25-user network isn't a big load on an Exchange server. (An Exchange server is a huge load on an administrator, or on a person who wants email to work properly, but that's another story.)

I currently run an Exchange (2003) server at a remote office with 35-ish people on a single Xeon 5160 (3GHz, dual-core) with 4GB RAM, and I've never had any load problems, and it is also an AD, DHCP, and DNS server.

When you say the remote offices connect via Citrix, will your Exchange server also be the Citrix server, because that will have a significant impact.

Exchange is more memory/disk speed hungry then it is CPU bound. Make sure to spring for better disks or more memory before you look at better CPUs.

And as Bitt said, CPU will be more important if it is also going to be a Citrix server.

True, but (IMO) it's rarely worth bothering with single-core CPUs these days. I'd probably opt for a 64-bit CPU and plenty of memory.

Another question: will you be running AV on the Exchange box? That adds extra load...

Citrix is on a different box, as is AD, we're going to be using Google postini for AV and spam filtering apparently:

http://www.google.co.uk/postini/compare.html

Oh, I totally agree. Honestly, I don't think that Dell offers any single-core CPUs any more. Even if they do, they're only worth it if you need raw speed out of a single-threaded process, and that's an unlikely situation, and one that an Exchange server is not.