The NAS units wouldn't benefit a great deal from hardware RAID anyway...other than for RAID5 writes. In some recent testing here a 3 drive RAID 0 array tested in at about 150MB/s (writes) using the motherboard's Nvidia RAID controller vs the Adaptec 3405 PCIe controller (4 drive RAID 0) at about 200MB/s writes. Had the Nvidia embedded test used a 4 drive array instead of three, the numbers would likely been similar.
Now if your NAS has one LAN connection, then 100 MB/s or so seems to be the gigabit limit and will be as good as it gets. For RAID 0 writes over the LAN (Vista SP1 workstation, Adaptec 3405 RAID 0 array, 4 drives, 2.5GHz core2duo), we're getting about 87MB/s measured using a 5.3GB file set copy. If that machine was using RAID 5 instead of RAID 0, we'd likely max out at ~50MB/s. Looking at various reviews, it would seem that going from RAID 0 to RAID 5 typically halves the write performance of the hardware cards due to the parity calculations/writes. Intel ICH7 (onboard) RAID5 writes were about 25MB/s and the Nvidia (680SLI) RAID5 writes were a dismal 10MB/s or so....so 50MB/s from the 3405 is not as bad as it sounds.
The TS509 already performs at ~50MB/s write, and ~85MB/s read in our tests. Update it to 4GB of memory and files under 3.5GB or so will write to it's RAM at just over 100MB/s. This may sound terribly confusing, but essentially I'm saying that for a Gigabit NAS unit like the TS509, (and similar units) hardware RAID beyond what they're using right now would likely not make a huge difference given wire speed limitations for typical use.
Now if you're copying 500GB files to the NAS, and you want two workstations to be able to do this at 100MB/s all day...forget NAS units and just build a workstation with a RAID controller, 8 drives, and dual gigabit LAN.