I'm in a similar position. I have an iMac and am considering replacing my Windows HTPC with a Mac Mini and want one copy of my music/videos/pictures etc rather than multiples floating around.
One device that seems to fit my needs is the LaCie Gigabit NAS. There are several Ethernet-capable HDD's out there but this one has the best overall customer reviews and has a USB socket for attaching a second drive to add to the storage. I was considering a 320GB drive for the NAS and a second similar external drive for Time Machine back-up.
On UK sites I've seen the 320GB version for around £115 and the 500GB version for £135. The 500GB drive would make more sense financially and could be partitioned in two but then if the drive should die you lose the original and the back-up.
Hope this helps