Actually, I would say that for a low-cost solution, that would probably be alright. The problems that I see are:
- Typically server clusters use a stripped-down OS (think Linux or a
BSD) with a bare-minimum of services
/ apps. Neither MacOS or Windows fit
this bill.
- You can probably get a better break on pricing going with another machine
type. Honestly, you can probably get
a laptop from an office-supply or
big-box retailer for a comparable (if
not cheaper) price. (...or NewEgg, or Amazon, or TigerDirect, or ...) Finding gigabit
network and 4GB of RAM should not be
hard. (While I love the Mac Mini, it doesn't sound like it gains you any real benefit in this case.)
Also, Mac Minis aren't Rack servers, even if you place them in a rack. (Yes, you can purchase rack-mountable shelves, but that still doesn't make the Mini a rack-server...) Rack-mount servers are a very specific chassis type. They aren't cheap; typically a rack-mount server will be more expensive than a comparably-equipped Tower server.
FURTHER:
If possible, it may be worthwhile to have one Windows box dedicated to pulling in the images (via your activeX control), dumping them into a share which is then accessed by a *nix cluster for processing. (Assuming that the downloading isn't the part that needs processing.)