If it's just a data server, I wouldn't spend too much money on the core hardware (CPU/Mobo/RAM); I'd spend it on fault tolerance if it will always be a data server. I suppose it depends on what you want to do, but even intense read/writes aren't going to use a lot of CPU or RAM. If you run something like FreeNAS, you can run it on old POS hardware.
I'd invest in a 1Gbps network as well, if you don't already have that.
You might consider looking for something that supports iSCSI. An iSCSI initiator comes with Windows 7, and it's a free add-on for XP and such. It would allow you to mount a volume in your server as a local disk. This is vastly different than mapping a drive. With iSCSI, you have direct access to the disk(s) just as you would if they were installed right in the local machine, so you can partition, format, etc...
I've seen some RAID solutions (hardware and software) that allowed mixing drives, but it is never recommended. They should at least have the same spindle speed and nearly the same capacity. You will forfeit the difference in storage capacity on the larger drivers. You also forfeit performance on faster drivers (i.e. The array will run only as fast as the slower disk.)