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. =)