While you don't have to match the RAM speeds and you can interchange them, it will just run at the speed of the slowest module. However, it should at least be notes that not all machines play nice with this. Some high end machines will not boot with mixed memory, and some will become unstable from doing it.
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This machine uses DDR2 667 memory, ideally. I'm sure you can put 800MHz in too, but it will downclock to 667MHz and you run the risk of stability problems.
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Either get a 667MHz kit, or try 800MHz in the machine and see what happens. You can see how to install the memory [https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Core+Duo+Model+A1150+RAM+Replacement/481|here].
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It depends on how specific you are about speed matching, and any potential problems. if you want it to work correctly and not have a problem you will want to match the modules exactly. If you just want the speed to match, you can buy a module of the same speed and match the speeds. These 2 options are what I suggest doing.
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If you aren't concerned about any of that and just want to get more memory, you can mix and match speeds and brands as long as you are okay with the new module taking the speed of the original module(s) that came with the computer. I don't suggest doing this but it is an option. The problem is doing this can cause instability problems. It doesn't always happen but the chance of a problem isn't exactly in your favor. It's better to match the speed if you can, or bite the bullet and buy all new memory modules. With that out of the way, here's the [guide|481|guide] to install the memory in the system.
While you don't have to match the RAM speeds and you can interchange them, it will just run at the speed of the slowest module. However, it should at least be notes that not all machines play nice with this. Some high end machines will not boot with mixed memory, and some will become unstable from doing it.
+
This machine uses DDR2 667 memory, ideally. I'm sure you can put 800MHz in too, but it will downclock to 667MHz and you run the risk of stability problems.
+
Either get a 667MHz kit, or try 800MHz in the machine and see what happens. You can see how to install the memory [https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Core+Duo+Model+A1150+RAM+Replacement/481|here].