You got a point pmorris, the Linkstation pro is also a nice option powered between 2) and 3)
Features: “400MHz Marvel Orion Media Vault processor • 128MB SDRAM • 250GB SATA hard disk”
means the CPU is about 1.5-2x the speed of the NSLU and most importantly its ram is x4 which should make it really significantly faster already.
At a price of 150 EUR including the harddrive it also fits pricewise between 2) and 3).
How easy is it to get unslung or similar on it?