If you are just looking for a nas box, then WHS adds quite a bit to the price, not only for the licence, but since it is based on Windows 2003 server, it ups the processor and memory requirements. (Of course, WHS is much more than simply storage)
If you do want to go down the WHS route, then (in my experience) if you want it small, quiet, efficient and upgradable, it won't be cheap!
I looked these options about a year ago, and eventually went for price and upgradability and ignored the other issues. (Partly because I wasn't really sure whether WHS would do what I wanted, so didn't want to spend big until I was sure.)
So, I got a cheap HP ML115 G5 Server (Don't know what the RRP on this is, maybe around £200 + vat, but I have several times seen special offers where it is sold a lot cheapear, eg below £120+vat) I then added two terabyte drives and a bit of memory. (Should point out, they I did have a little bit of problem with this machine due a driver issue which affects some hard-disks. I originally want to leave the operating system on the supplied 160GB drive and use my terabyte drives for storage, but couldn't access the small drive and so I give up and just install the O/S on one of the big drives).
Regarding storage capacity, since one of the features of WHS is balancing data across hard-disks (doesn't balance the operating system), you may want to double your storage requirements. (This feature which can be turned off, but you lose an important benefit if you do).
You might want to browse the forums at http://forum.wegotserved.com/.