upgrading ram

I have two ram slots in my pc with one 256mb ddr sdram. I can either buy 2x512 ddr or a 1x1gig ddr and wait to buy another 1gig stick later. my mobo and graphics card are both old and at minimum specs for winxp. I can see at least a four month wait until I could buy the second 1gig stick.
Will the 256ddr i have already work ok with the new 1gig until i could replace it with another gig?

make sure the memory you get is the same brand if you run them together
as well as the same speed
also be sure you put them in the correct slots if it is dual channel