It all comes down to what you want to use it for...
Generally speaking, two disks are more complicated to run (setting up sync jobs / RAID/...), and consume more power than a single disk solution. You do however get the advantage of redundancy, which for certain use cases is a must-have. I am personally a touch skeptical regarding multi drive NAS products, because most manufacturers don't detail how the RAID can be restored in case your system (not the disks) dies.
The questions you need to ask yourself (in no particular order):
1. How many users will be accessing the system (more concurrent users == you want more Cache/RAM/CPU)?
2. What are your future requirements as far as space is concerned (no point in buying something that is at 99% straight away).
3. What kinds of files are you going to be storing on the machine? (big files like movies for streaming clients don't require performance; editing thousands of photos over the network means that you need some serious caching (RAM) / faster disks on the NAS).
4. What "other" features will you use / need? If you just simply need SMB, then you're fine with pretty much anything. If however you're wanting to run a BT Client, Mail Server or whatever, then you will want something with slightly more advanced firmware, and most likely a more powerful CPU.
There are two main ways to go about a home NAS. Either buy something off the shelf or build something yourself. There is a rather large selection of good off the shelf products: I can at least recommend the newer QNAP's, WD Live series and newer Buffalo systems. My advice is to try and go for atom CPU's here- they are much faster than their Marvel equivalents. The Marvel based systems are fine if you're just streaming music or movies to a client, but no fun if you need performance. The Intel/AMD based systems typically provide a much better experience with larger numbers of concurrent users &/ "fancier" features like mail server etc.
The other option of course is to build your own. Here you can either take your old machine from the garage, or get a shiny new mini box with low power cpu (mine is the HP Proliant N40L Microserver). Whack in a couple of HD's for storage and a USB Stick for the OS and you're away. Here you can either go with a relatively simple system like FreeNAS, a standard Debian/Ubuntu/Fedora or splash out on a Windows Server / Home Server License.
Check out http://www.smallnetbuilder.com/
for some pretty good reviews of most current systems out there.