Its the number of Mobo / processor options you have which does my head in.
I recommend that you choose the CPU first, that then dictates the mobo by socket type. Then choose the mobo by form factor, expansion slots, features, reviews, manfr.
I would want to buy a quad-core CPU at this point, AMD Phenom for example (I like to support AMD to preserve some intel competition).
Look at ebuyer and lambdatek websites.
If you want 3 or more HDDs, you'll need at least a midi-tower case, plus a good big PSU. So an ATX FF.
SD