generally speaking mixing RAM is 'possible', but not recommened. When mixing RAM, consider the voltage requirements for both. You may find that one module requires a higher voltage than the other. If this condition is true, verify that the module requiring less power can be used at the higher voltage.
Also you should know that when mixing RAM you will be forced to use the higher latency of the mixture and at the lowest speed.
1 x 1GB 400 MHz 1.5v 5-5-5-12 paired with 1 x 2GB 800 MHz 1.7v 7-7-7-15, if they can be used together you'll have the physical 3GB, but you'll be operating on the lower speed and the higher latency.
Furthermore, when using Dual Channel, if you don't use a matching pair, you will likely lose some speed. This is because a lot of motherboards will map the lower matched RAM into dual channel and the remainder of the unmatched RAM will be mapped into single channel.