You are basically asking a question that is dealing with two scenarios: more memory vs faster memory.
It has always been my recommendation to go with faster memory.
In your case, you are dealing with two different memory types. For many motherboards, the memory pair might not function properly, and could even create problems for your system. This is definitely a 'down'
Many applications do not require more than 512MB to run, so going with faster memory will help you more in most cases.
On the other hand, if you have a slow system, i.e. slow system bus, then it doesnt really matter too much what memory you put it; it will just be slow.
So yeah...
My recommendation: stick with faster identical memory pairs; go with the single stick of DDR400 (PC-3200)