Check out
www.corsairmicro.com to check for motherboard compatability. Also check to see whether the automatic timing settings are appropriate to your motherboard, otherwise you will get the "blue screen of death" associated with windows memory failures (and a lot other besides).
You will not be able to run the two memory banks in true DDR (I'm assuming you were referring to a single memory stick with 512 on and not two paired sticks here) but assuming you comp supports it you could run these in seperate memory banks.
Far better if cash allowed it to buy the Corsair TwinX modules which are paired modules guarenteed to work with DDR memory isn't that expensive these days, and the increased performance from using DDR is worth it, especially as you would be underclocking a more expensive component to get a lesser effect
Visit the corsair site anyway though, and browse through the memory tutorials that they have there, it will be well worth your time in informing you of current memory technologies.
Ransom