Heya,
I totally understand the small footprint thing. However, you're still going to quickly discover the limits of a pure NAS device, especially one that is not gigabit network capable, and especially one that has limited and/or frustrating operation. My suggestion to folk is to build their own out of a computer. However, as you stated, you want small foot print. So to that, I say look to the micro-ATX towers. They can be fitted and modded to hold 4 drives. You can control what goes in and you can do so for cheaper than a pure NAS device. You can ensure a gigabit ready little machine with powerful yet cheap components and you can choose your own way to serve, be it FreeNAS or something like Windows Home Server. Completely headless either way. That's what I'd do if you want really low foot print without buying a limited pre-build NAS device that will not grow with your needs.
For example, look at this case:
Steel MicroATX Mini Tower @ $39
For $39, you get a case with a little PSU already in it (this is more than enough to power a little CPU and hard drive setup). It's MicroATX so a cheap little mATX board will fit in there. It has 4 drive base for 3.5" drives, your HDDs. The thing is only 16.5" deep, 7" wide, and 14" tall. It's literally the size of a little battery backup that people buy. It's the size of shoe box. $39. And has 124 reviews with a 5/5 egg rating. That's cheap and fantastic.
Note: You can use 5.25" to 3.5" converters to make those large bays house more 3.5" drives. You could easily put 6 HDD's in that box via a little cage like that, 7 to 8 if you mod it, but easily 6 with just a HDD cage for the 5.25" slots. And you could still have room for a DVD drive if you wanted for installation or backup purposes...etc. If you went that route, grab a motherboard with 6 SATA ports (or get any motherboard with 4 ports and get a cheap little 2 to 4 port SATA controller card (PCI/PCIe) in there. Here's a link to a little cage to let a 3.5" drive be housed in a 5.25" slot (to use those two slots on the case, to make it house 6 internal 3.5" drives easily, with room for a DVD drive still:
3.5 to 5.25 HDD cage.
Just add the components to make it tick:
FoxConn A7VMX-K @ $55
AMD BE-2350 Brisbane @ $45
2GB (2x1GB) DDR2 800 Memory @ $20
And that's all you need for the basic NAS machine. It's got built in gigabit lan, built in audio (just in case), built in video (handy if you need to do more with it), enough components to last you a very long time into the future in terms of serving files around and running any OS you want essentially. You can even RAID with it if you felt the need (I wouldn't RAID a NAS though anyways unless it was for more than casual use).
That's
$160 for a shoe box sized NAS that is excellent. Plus, if you outgrow the machine, it's actually a fine little computer that is ready to be a casual ready machine (lan, audio, video, etc, built in, ready to go, powerful enough to run Vista even, it's a dualcore after all). A limited specific NAS device becomes a paper weight when you out grow it's use/need. This little thing can be your friend's machine, or a backup if yours died, etc. Really great way to get multi-function and future use out of a machine you're putting money into. Especially only $160.
Just add drives.
Very best,