So, I'm now considering moving to a Windows Home Server solution instead. Everything I've read says it can do all this easily, plus the transfer performance is close to what you'd expect on a GB LAN.
WHS transfer performance will actually depend on what hardware you're running WHS on - so don't let anybody tell you what to expect, unless you give them the hardware spec you plan to use, or they give you the hardware spec their performance numbers were achieved with.
I was a part of Microsoft's WHS public beta and the transfer numbers I saw with a Realtek gigabit NIC were mediocre beyond belief, even worse than my DNS-323, in fact, not even up to 100 mbps levels - I tracked the problem down to Microsoft's native Windows drivers for that particular NIC and was able to double the throughput by using the drives that shipped (on a diskette) with the NIC, but, at it's best, that box could not transfer data any faster than my DNS-323.
Yes - I was running the beta on older hardware that barely met the hardware requirements for WHS, and would no doubt have seen better performance had I used more capable hardware, but then that pushes the price up, and the power consumption up, and guess what - my DNS-323 does everything I need, include backup my PCs and without loading any software agents onto the PC as is required by WHS.