See this article
This indicates that it is possible to go to 3Gb of RAM in the mini.
Remember though that Apple will not support such a configuration and any problems you have will not be addressed by them until you go down to the supported configuration.
With RAM prices as they are, it's worth a try
Ok people, I'll give you the lowdown. I bought a Mac Mini about 6, 7 weeks ago now, delighted. Leopard is a fantastic OS, but with the stock 1GB of memory, I find that running a Virtual Machine, Safari, and a few other small apps, things can become a little slow. Now, I've read that these Mac Mini's have a limit of 3GB of RAM (Apple says it's two) but I don't understand how that works if they only have 2 RAM slots.
I intend to purchase two sticks of 1GB (RAM is insanely cheap these days) but if I was to purchase 2 2GB sticks, would this make the 3GB max?