Im still a bit confused...

I want to add more memory to my system, but I am worried about causing instability. Do I need to order the exact same RAM to run everything smoothly or will a less expensive alternative do just fine. Right now I have 2x512 and only have 1 more open slot. I don't know if I need to get another 512 stick of the same stuff, or if I could maybe get a 1gig stick of some cheaper pc3200 stuff. Can you please help me?! Thanks..............

To my knowledge you would have to buy identical RAM. Also if you only have 3 slots, I think putting another stick in the 3rd slot will slow your ram down to 166 instead of 200.