spekesoftly,
If you have WinXP, then by definitely go with 512Mb. If you are running another OS let us know which. Each OS has its own sweet spot for RAM.
Here is your motherboard:
Take Care,
Richard
P.S. Here is a note by one the MSI Engineers about the chipset:
As 845 chipset does not properly support the PC100 memory, it is strongly recommended to install PC133 DIMM modules for better system performance and stability. We do not guarantee the system stability when installing PC 100 DIMM modules.