One possibility....

...is to consider a dedicated Linux server. You can buy components according to your needs and budget, upgrade at any time without much trouble and without buying a whole new machine, you can have RAID 6 without hardware (I recomend a good Raid card though, go Areca, will cost you a bit but you get great bang for the money) and you can share files with Macs using Netatalk and with Windows clients via Samba. A little server for two seats shouldn't cost you more than 1.000 bucks (wo. hardware raid) with plenty of hard drive space.
Downside is you need to know a bit about what hardware to pick and how to configure things.
You can even go down the 10Gig lane one day with a multiport nic for the server and cards from Small-Tree for you Mac clients. Or iScsi, or Fibre. You are very flexible there.