Depends on the layout, CAS latency, number of chips, etc, etc.
I doubt you know all the latencies, but your lowest CAS latency chip needs to be in the first slot. Secondly, adding 256 to what had to be 768 probably isn't going to help. I'm guessing you now have either 512 + 256x2... do yourself a favor and sell the 2 256 chips and buy another 512. (I really hope you're not using 4 256 chips).
Rule of thumb: adding RAM is usually a good idea, as long as the increments are in doubles and you order them correctly.