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Actually, the older MBPs will support 6GB. I have 6GB running just fine on my 2007 2.4 MBP. MBP 5,1 will run 6GB max. 5,2-5,5 will run 8GB. Apple claims it will only support 4GB, but the hardware will do more and OSX will use it just fine.

Thanks for confirming that, I wasn't so sure anymore.

OP, you can use www.everymac.com ti check out specific technical data on Macs, it even lists the maximum, unofficially supported RAM capacity.
 
If the size of the two sticks don't match I'm pretty sure you'll lose dual-channel capability.
 
Thanks for confirming that, I wasn't so sure anymore.

OP, you can use www.everymac.com ti check out specific technical data on Macs, it even lists the maximum, unofficially supported RAM capacity.

I was a test user for that around our office. We had a bunch of 2007 era MBPs, and had some users complaining they were "slow". I personally think they were just jealous of the new unibody designs. I found the tests on Macsales.com and showed them to my boss, and he bought some upgrades for us (including his). To be honest, the only time I noticed a big difference was when running VMs on it. But it's still nice to have it and not need it.

Oh yeah, Everymac is a great site.

Edit: losing dual channel isn't that big of a deal. OWC ran some tests with and without dual channel memory, and it didn't make much of a difference. They basically found that the benefits of having more memory outweighed the benefits of less memory with dual channel. I'll try to dig up their tests.
 
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