Can't Install Mac os x on my MacBook.?

I'm trying to do a fresh install of snow leopard, but after booting from the DVD and choosing the language a message pops up saying "Mac os x cannot be installed on this computer". I have a 2.1ghz intel core 2 duo white MacBook it is currently running os 10.6.6. I have verified the hd and checked that it is a guid partition table. Although I did swap out the memory sticks from an acer laptop a while back they run at 667mhz the same as the originals ones and I did a hardware check, and theyre fine. I don't have the original ram but could this be the problem, what else could it be??? Any help would be much appreciated.