Well, thank you for the kind offers, but part of the desire to build a server is to trim down on the power consumption. Can't see raid arrays helping with that

Currently it's a pretty barebones P4 2.8 machine, which draws 50W at idle and 80W under load - so the yearly leccy cost is something like £65. That's more than I spend making 20x 125l batches of biodiesel at 16kWh a batch!
So I'm considering either a 18W GA-E350-USB3 (nice bit of kit, integrated board and dual core processor - but it might be a bit underpowered), or a suitable dual core Celeron like this'n:
http://www.scan.co.uk/products/intel-celeron-dual-core-g550t-1155-sandy-bridge-m-dual-core-22ghz-2mb-smart-cache-oem...which is 35W vs the P4's 65W.
Trouble is, the payback period by the time I've added memory, mobo, sata disk, PSU and case is up at the 4 year mark so is it worth it?
I couldn't take any donations because our little server is used for lots of other things than websites, namely content filtering for the kids, CCTV, being our mailserver - the webserver bit is just a sideline really.