the motherboard lists all three timings to show what it is tested to work with.
For example a motherboard lists below
It means that the motherboard can work with all types of RAM rated at any of the speeds mentioned. However if you buy 1866 RAM it means that you will have to overclock for it to work. I.e the RAM has been tested to work at that speed on the motherboard but you are going beyond specifications and it may cause problems.
Another general rule to know is that RAM can always easily run slower than it's rated speed. So let's say that you have a stick of 1066 RAM and a stick of 1333 RAM, the 1333 can be underclocked to run at 1066 for the sake of reducing latency etc. But with DDR2 and DDR3, RAM is always best used in pairs or triplets depending on motherboard hence the selling in kits.
well bigbluedart, you sound like you will be using old RAM on a new system till you get the funds to get new RAM. The few scenarios I think could happen is one, the RAM won't work at all in which case you just wait till you get the new sticks, or if you're lucky the RAM will work. This is assuming that your old sticks of RAM are compatible. DDR2 RAM on a DDR2 motherboard etc.
If the RAM speed is slower than what the motherboard officially supports, it could work if you play around with the BIOS. There is such a thing as underclocking after all. =)
The board I'm getting only lists down to 667 memory (DDR2). I have the next step down at 533 (also, DDR2). So I guess if I'm lucky it will just work. If I'm not I just get to do a bit of BIOS tweaking. Hurray!