You can use a program called diskwriggler to get speed info. Search the forum for how to use it. Typical speeds would be 6-10MB/sec.
NASLite 1.x only sees on board controllers, so adding a controller card won't work. When you upgrade to 2.x you can use the add in controller.
You didn't mention what ethernet speed you are using. Upgrading your network to Gigabit ethernet can increase the transfer rates, but usually the ATA interface is the limiting factor. If the ATA speed on both motherboards is the same then you probably won't see much of an increase. NASLite doesn't gain much from going to a faster processor. Your 533MHz and 256 MB ram should be fine.
Upgrading to NASLite 2 and using your ATA 133 controller will provide the most speed increase if your motherboards aren't already ATA-133 speeds. Then the limit will be how fast your drives can get data and your network speed. 100mb ethernet will still limit you to around 10-12MB/sec transfers. With Gigabit ethernet you will get whatever your disk can keep up with