To the op, I was confused about the mismatch ram as well when I was looking into minis. I found a thread that tested the mini with matched 1g-3Gs. It showed that the performance improved even though they were unmatched. As everyone saying more ram is always better. Heres link, maybe it can help.
"The bottom line? The results of our testing did not support Apple's recommendation and rather make it very clear that in addition to the potential economic advantages, there are real performance advantages to having more memory - even if it means not having a pair. It is our recommendation to upgrade memory, be it one at a time or with a pair, or to 4.0GB - based on what you need and what your budget allows."
http://eshop.macsales.com/shop/apple/memory/Mini_Intel_Memory_Benchmarks
"The bottom line? The results of our testing did not support Apple's recommendation and rather make it very clear that in addition to the potential economic advantages, there are real performance advantages to having more memory - even if it means not having a pair. It is our recommendation to upgrade memory, be it one at a time or with a pair, or to 4.0GB - based on what you need and what your budget allows."
http://eshop.macsales.com/shop/apple/memory/Mini_Intel_Memory_Benchmarks