I asked how much I should get. Andru answered:

Figured, I’d drop you a piece of advice as far as the RAM goes in the new Penryn-based Mac Pros (which I have.) I saw you were talking about having 2, 8, 16, or 32 GB in there. However, you aren’t limited in that fashion… not sure why you think those are your only options. Your RAM needs to be installed in pairs, and you get up to 4 pairs. Those 4 pairs do NOT have to match each other. I have 3 pairs of 2GB modules and 1 pair of 1GB modules in there, totaling 10GB.

I got mine with 2GB, and had ordered 8GB separately. The RAM came after the computer did, and I was in hell for 3 days while waiting for that RAM to arrive. I keep an absolute minimum of 14 apps running at a time on this machine, including Safari which at this moment has 4 windows open with a total of 37 tabs between them.

Anyhow, don’t even consider 16 or 32 GB RAM, it will be a waste of money. OS X is amazing at managing memory. I added the 8GB into this machine, on top of the 2GB it came with. 10GB total, and with all my apps running (including Vista running in a Parallels VM with 2GB RAM dedicated to it) I have 3GB of RAM free, and my swap memory on my hard drive has not been touched at all, once.

So if you plan on running a VM, as well as a ton of other apps at the time time – like I do – you will only need more than 10GB if you plan on doing more than I do at the same time, which is pretty rare ;) If you absolutely want to have all your RAM slots filled, go for 12.

Just my two cents.

Well, his two cents may have saved me $2,000. Now, it’s just figuring out where to go for the Mac Pro RAM (and no, I’m not going to Apple for it)… and deciding how much I’m really going to get.

I’m really leaning towards 16GB for forwards compatibility, but wouldn’t 32GB give me bragging rights? :)