I'm going to be putting Smoothwall/IPCop on it so initially I'm only going to be screwing around with the firewall capabilities, but eventually I'll most likely get into FTP, Proxy, DNS, HTTP, etc etc. Prettymuch whatever the rig's hardware lets me do.
Well you certainly don't need much in the form for cpu. If your building this mostly as a testbed for experimenting with software then you might want to look for a barebones server bundle. Single processor systems usually the cheapest and you tend to find the majority of those as xeon systems. If you watch newegg well you can sometimes score a xeon system with the cpu thrown int the bundle for around 500$, all you need to add is memory and a hard drive. and your set.
Another note if this is mainly gonna be a test bed and you want to invest in a server class cpu. Check out VmWares ESXi server. Base version is free. Its a server hypervisor, it installs with ease on compatible hardware. It allows you to run a number of installs and then access them inside the VMware vSphere client, which means you can put the server anywhere (perferrably somehwere quiet) and you only really need to plug it into the network and have it set the the correct network, and then all the visual and inputs for desktop control are handle through the vSphere client. Super big time saver, allows you to make great use of the cpu cycles and ram you have in the box, aswell as storage. Beats building a new box everytime you want to test an os install and stuff like that.
ooh quick break down of processors.
Xeon 3000 series single processor systems, fits in some i7 boards aswell. Dependsing on series number
Xeon 5000 series - these are dual processor capable processors, for systems with two physical cpu's
Xeon 7000 series - quad+ capable processors, for systems with 4- 16max(i think 16 is the max not 100% sure) physical cpu systems. Most expensive xeon, the boards cost a ton aswell. (1k-4k pricetag for some of these chips)
Opteron 4000 series, single to dual physical processor systems.
Opteron 6000 series, dual to quad physical processor systems.
In most cases the max ram for a single processor system is currently 24-32gb of ram with 64gb being the limit most probably by the end of the year on the newer xeon cores.
Dual processor systems are currently capable of up to 96-128gb of ram. This is where the extra IO channels for the larger server chips start to come in handy. As the physical cpu's go up the ability to handle more memory goes up aswell.
Quad processor systems are capable of 512gb to 1tb of ram depending on the motherboard.
If this isn't enough information to help you let me know.