I think the command "sync" should do it, but no gaurantees. There was just a lively discussion about this in the newsgroups recently, where someone wanted to test the read-write speeds of a hard drive, and found that sync will sometimes pull a read out of the buffers, if it still exists, so he wasn't really reading from the drive. Talk was that he needed to flush all unused ram to force it out , and something about "future kernel version, etc. :-) )

But in your case, I think sync will suffice.