It's true, and not just for Vista, but for all computers.
However, you seem to be confusing speed with capacity.
If you have 1x512MB stick of say DDR-133 RAM, and 1x1GB stick of say DDR-100, you will end up with 1.5GB of RAM running at DDR-100 speeds. RAM modules can have different total capacities without any issue. So you could have say 1x256MB module, 1x512MB, 1x1GB, and 1x2GB and you'd have 3.75GB of RAM, all running at whatever speed the slowest module is.
There's a bit more to it than that, but that is a good overview of the topic.
If you had a laptop with 1 512mb RAM chip and 1 1GB RAM chip, is that laptop only really going to be running as fast as if it had 2 512MB RAM chips?
In other words, the 1GB RAM chip only runs at 512mb because of the other 512mb chip. True or false?

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