[markw10]

I have one of the previous generation MacBook Pro's, a 15" 2.33 Core 2 Duo. I always have heard that the maximum memory that you can use in these is 3GB, that you could put 4GB in and it would work but would only use 3GB and if I remember right report 3GB in the About this Mac section.
I have been using 3GB but since a 1GB memory stick reported errors I got a replacement. I read that if you use a 2GB stick it would work as 1GB but work in dual channel mode so would be slightly faster. Since I got a good rebate I ended up buying a 2GB stick and it is 667MHz DDR2 SDRAM.
I was shocked to boot up my computer and see it reporting 4GB. Since it's reporting it is it using the 4GB or just reporting it as 4GB and only using 3GB? I know I read all of this when Tiger was out so is this because of Leopard that it's maybe working as 4GB?

[mfbernstein]

It's a hardware limitation. Leopard/Tiger make no difference. Intel's Napa chipset (inside your MBP) can only address about 3.3GB of RAM because the upper 700MB of address space is used for devices on the PCI bus. The system may 'see' 4GB, but you will only be able to actually use the 3.3GB that are addressable.

[markw10]

Okay, that makes more sense. Well I guess at least I got .3GB out of that extra gig so it's probably worth it. I do a lot of multitasking and running Windows in VMWare so even that little extra should help.

[Rev2Liv]

I also have a previous gen. 2.16 C2D MBP with an ATI Radeon X1600 chip. I came across the same issue you did.

A smarter more observant member pointed me to the Activity Monitor where in 10.4, it reported only 3GB being addressable,

[Simon]

Yeah, what you can do is open Activity Monitor and check actual RAM usage.

Even if your Calistoga (that's the chipset not Napa, Napa is the Centrino platform) MBP reports the full 4GB you can see your system is using a lot less.

Addressable RAM = Used RAM + Free RAM
where Used RAM = Wired RAM + Active RAM + Inactive RAM

Check Activity Monitor for those figures and add the numbers. In a Calistoga MBP like the OP's, you'll see that the total is 3GB which is the addressing limit. Although the MBP sees it's installed, that last GB is simply not available to the OS.

The performance advantage of having a 2nd 2GB DIMM rather than a 1GB DIMM comes from dual-channel memory access. However only very few tasks are bottlenecked by raw FSB bandwidth. A good example where people see a benefit from the matched DIMMs are MBs with integrated graphics where all the communication between the GPU and the video memory (which is actually in regular RAM) has to go over the FSB. In such a situation you can see roughly 15% performance advantage for matched DIMMs. On a MBP there is dedicated VRAM so this bottleneck doesn't exists and the performance advantage of matched DIMMs is almost zero.

If you get a good deal on a 2GB DIMM (like the OP did) 4GB certainly won't hurt. OTOH if you can get a 1GB DIMM easier or for less, there's no reason to go for a 2GB instead. There's more information in this discussion here.