Okay to mix memory??

I currently have a pair of Corsair TwinX low latency pc3200 2x256 in my system, but want to bring that number up to 1 gig. Is it okay to mix types of RAM? Say i put in a kingston pc3200 512 chip in there, or a Corsair Value Ram pc3200 512mb in there. If I am able to do that, what are the stipulations/drawbacks in doing so? Thanks guys..

its okay to mix cake batter but not so good to mix memory. u can probably get it to work, but it will only run the speed of the slowest/highest timing stick...