I'd say it's probably best to build your own NAS... That's what I did, and I was deciding between doing that and buying Intel's 4-bay NAS for ~$280 that had an integrated Celeron.
I went with building it myself because that way I'd get the most flexibility. You can build it and have it run Linux (mine runs Ubuntu), and whatever client you want (I use KTorrent) that will run on the OS you choose.
In the end, the custom build cost less than $350 (Intel Atom integrated into a board, RAID card, 1GB DDR2, 500GB boot drive, case with included PSU) excluding the four 1.5TB hard drives I threw in. They run in RAID 5. I never thought the D-Link NAS builds were ever worth it - 2-bay empty NAS for ~$350 or 4-bay for ~$650 just weren't worth it.
Good way to learn Linux too. I was pretty new to it at the time, only having worked briefly with Linux machines before, and being almost a complete n00b.
If you were to ever buy a pre-built NAS, go for Drobo. It's the only one that's really worth the money because of its features. But be warned, it's quite expensive.
