Well, the reason for using a server is that all your machines can access it and you only have to install everything once; plus if one machine goes down, you just reach for the next one (and hopefullly you back up your server [:)] ). I don't have one set up, but the idea does make a lot of sense. By the way, my network has never failed, in fact I've never heard of that happening, so I'm not worried about going over it instead of directly to the SATA bus.
As to Windows machines, I have two of them (plus a third that's not working) and it's not like I'm afraid of them - in fact I like them - but they can be a nightmare when something goes wrong. I mean, if you start up from a system CD, Windows wants to replace your whole system drive, programs and all! So for me the idea of my Windows machines being Macs when I need to troubleshoot them is very appealing.
On the other hand I've posted several times that I don't think Mac Minis are a good choice for sample-streaming machines unless you have an extra FireWire interface sitting around. They have no slots and only hold 2GB of RAM, and the built-in digital interface's two outs isn't enough. Plus you can only clock them through TOSlink, which is not an audiophile way of doing things (lightpipes are jittery). So I agree - although I think Mac Pros would make excellent Windows machines; if you need that kind of power, you can't put together the same thing for the price if you buy the equivalent components on the internet. I checked a few months ago.
Just out of interest, what $700 machine would you put together? Motherboard, processor, etc.