Run mixed ram on several different systems over the years, some like IBM's don't like it others, usually home built pc's don't seem to mind as much. As leggo rightly says it will all default to running at the lower speed but shouldn't give you any headaches. I've got bucketloads of ddr @ home so if you get stuck give me a shout.
The problem with RAM comes when mixing types i.e ecc and non ecc, this will cause instability so don't do it.!!!
Good luck!!