Are you running Windows 2000 or XP? If so, open up the Windows Task Manager and look at how much memory is being used. How big is the PF usage? Is it more than you have physical memory? If so, you are paging (swapping) and paying a performance penalty. If your PF usage never goes higher than physical RAM capacity, then more memory won't help.
I haven't looked at the details for your mobo, but many will work faster with 2 identical sticks of memory than with 1, by doing parallel memory access. Some mobos will ONLY work if you install memory in matched pairs. Since yours works with only one stick installed, it obviously isn't running a matched pair. Perhaps you'd get better performance with 2, but check the manual to see if that's true.
Also, use Windows Task Manager to see if you are CPU bound, and if so, what processes are consuming the CPU. Perhaps there's a pig process that you don't even realize is sucking down resources.