First off, this is the 32bit limitation with some BIOS' and OS. If you are running XP, you need to have SP1a installed in order to overcome this (it enables 48bit addressing) and it needs to be slip-streamed in your install disk.
Second, your box should support up to 2Gb of ram according to Shuttle http://www.shuttle.com/share/product_da ... /SS51G.pdf
So I have to ask again, what OS? If you cannot see all of your memory, either the memory controller on the motherboard doesn't like your memory, or, you are running an older OS (Win98) that has a limit of 768Mb of Ram it can address. This is a physical limitation with the OS.