i have this problem about 3 months ago, fine a pair that work together and sell them on (ebay is always fun) and with the cash you get buy a matching 1gig DDR sim to go with your other 1gig sim. then you should be able to put the matching pair of gig sims into one set of the colored ram sockets and the other 512mb in the other and this should enable dual channel also.
advice for the future, always get matching pairs. saves you bang you head against a wall
