The CPU on the Intel SS4200 NAS box is the bottleneck if you plan on using software RAID, especially within Windows. I upgraded the stock CPU from the Celeron 420 to the Pentium E2160, which adds dual core capability, double the cache, and a slightly faster processing speed from 1.6 ghz to 1.8 ghz. After running the upgrade, the system was a lot more responsive and file shares would appear a lot faster than before. I was planning to replace the SS4200 with a HP Proliant micro server, but I think I can wait after upgrading the CPU.
You can easily find used E2160s on ebay for $25 or less shipped. Another option would be the E2200s, which runs a bit faster (and uses more power) for around $30-$35 used.
The upgrade was very simple. A clear plastic cover needs to be removed along with 4 screws that hold the copper CPU heatsink down.