Like Phoienix32 said the bottle neck is in the cpu. Write speeds should be slower than reads. Use a FTP program to measure you speed. It elimiates some of MS Overhead. Speed transfer can be impacted by your network setup. There are some routers that have very poor performace.
If you look at all of the NAS out their all the good ones have some serious HP under the hood. Because doing XOR parirty calculations require it. My 4500 is equiped with a 2.4 ghz P4, 800FSB. My bottle nec is that my PC's are not up to the task to tax it. I am going to have to find program that can generate the file it is sending. Removing the HD (pc) form the equation.
I just did some speed test on my system using FileZilla FTP and SmartFTP. FIleZilla is 33% faster than SmartFTP, weighing in with a write speed of 15.8MB/sec substained speed. This was run over a gigabit switch. Test is in another section of the forum.
http://forums.procooling.com/vbb/showthread.php?t=13765