I just got my hands on the HP EX470 Windows Home Server Box, which has Gigabit ethernet [esssential for such a device, USB lke the DROBO has, is simply not up to the task], a single 500G HD and the option to add 3 more internally plus another four externally via USB and it also has an ESATA connection too. So I intend adding 1TB drives to rack it up to 8TB and that's without using the ESATA.
No point buying the EX475 version as it just has two 500G drives instead of one and I regard 500Gs as too small.
I was ready to install WHS myself, but found out there's a massive bug in it.
I would strongly
advice against using Windows Home Server for anything mission critical, or especially as a backup. There's a well-document bug in its implementation, which might or might not be fixed in the coming months.
MS has acknowledge this critical bug and is "working on a solution":
edit: DarkPenguin beat me to it - but the warning remains...