RAM upgrade must work on current and future system

I have an AXP1800 (palomino) system (see sig) that has served me well for a couple years. It is suffering however, and will be upgraded soon....but not yet.

For the moment I want to upgrade the RAM with a stick that will serve me well in the next system (mobile barton o/c or possibly A64). Is it as simple as buying PC3700 (or 4000) and plopping in alongside my PC2100 RAM? In the past I've had numerous troubles combining RAM sticks, and while I am far more versed in such things today, I still have that lingering doubt.

How should I go about establishing timings that will work for BOTH the new and old RAM?

In theory, PC3200 should work quite happily alongside your PC2100 RAM and will be useful for in a future AMD64 or P4 rig.

yes it will work. As long as the 2 sticks can run the same timmings at the same speeds.

Yeah getting the right timings is the tricky part.

For best future proofing, a good stick of TCCD would be best, like OCZ PC3200 rev.2.