Looks like that is this board: http://www.msi.com/product/mb/K8NGM-V.html#?div=Detail
Which says this:
• Supports 2 slots for 184-pin DDR DRAM DIMM modules.
• Supports DDR 400/333/266.
• Supports a maximum memory size of 2GB for DDR400
Meaning you should be fine running 1 - 512 and 1 - 1gb as that adds up to only 1.5gb and it supports up to 2. As long as they are DDR 400/333/266 it should work.
I will note that it won't run dual channel in that config - so the memory throughput will be a little lower than if they were matched sticks...but in this case I doubt you'd notice at all.