Unless you are running dual channel DDR you can stuff anything you want into any machine. It will just run at the speed of the slowest stick. (Exception, some machines require you to use buffered RAM if all 4 slots are populated.)
Some stupider bioses may lock up if you put a fast stick in the first slot becasue they don't look at the speed ratings for all the sticks. Just put the slowest stick in the first slot if this happens.
If you really want to keep the older RAM in there (I didn't, though my old was XMS 2400), just pair the DIMMs appropriately. AKA, don't put a DDR 2700 and a DDR 3200 DIMM in the same bank, keep both of the same type/speed in their own respective banks, and then it should go OK, albeit slower.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS