From playing with the Drobolator it seems like the best strategy is to upgrade drives at least in pairs. If I have three 500GB drives and add a fourth 1TB drive, only 500GB of the 1TB one will be used. Presumably because it has nowhere to mirror the other half. Another side effect might be that speed becomes variable; with 2x500GB and 2x1TB is seems like there's essentially two speed layers: one consisting of the first 500GB, which is spread across four drives (3/4 data and 1/4 parity), and one 500GB layer which is spread across two drives (1/2 data and 1/2 parity). The half of the 1TB drives that simply mirror one another is, I'm sure, not going to run at full tilt. So at least upgrade in pairs, preferably all four for performance.
So what I've done is bought 4x500GB drives (for $300) for 1.4TB of usable capacity. When it fills up I'll see if I can get 4x1TB for $300 to double the capacity, and simply swap all four out.
Got the drives sitting here... waiting for the Drobo itself. Lead was 5 days, but now is posted as 7 days, so... who knows.
Got the DroboShare already though, as it was available for immediate shipment. It only uses USB however, so is not likely to get more than 30MB/s or so, assuming their software can make good use of it. The good news about USB is it'll be easy to unplug the DroboShare and hook up the storage itself directly to a computer.