In my "vintage" 2x3GHz Dual-Core Intel Xeon MacPro, I have two pairs of 512 MB DDR2 FB-DIMM, occupying the slots labeled DIMM 1 and DIMM 2 in each riser.
For general FCP/Photoshop/AE work, I'd like to add RAM, although I don't really notice any performance issues at present.
1. At the crucial.com website, 4 GB (2GBx2) costs $390, 8 GB (4GBx2) costs $1570. It would be cheaper to buy two 4 GB kits -- any problem with that aside from using more slots?
2. If I bought two 4 GB kits, and added them to my 2 GB (which currently occupy two slots in each riser), I would have a filled-out stack of a total of 10 GB. I have read that filling out the stack is a good thing -- true?
3. I think that my RAM sticks are ECC -- am I correct?
1) Be sure to get out the computer's manual and follow it exactly on how to install it. They have to be put in in the exact right positions, no exceptions.
2) It should all be ECC, but you can trust Crucial's site. If they say it's right, it is.
3) RAM has to be in pairs, which you seem to know. As you suggest, you can get away with odd matchings of different kinds of pairs (like 512 x 2 + 1 GB x 4 for a total of 5 GB). The computer can deal with that, but Final Cut Pro has stricter guidelines. It wants ALL the RAM to match. Dunno why, but that's what Apple says. It'll work on that hypothetical 5 or 10 GB computer, but Apple just says you could have problems, whatever that means.
So buy 1 GB or 2 GB sticks from Crucial and REPLACE what's in your computer. I suggest either 4 GB total (1 GB x4 OR 2 GB x 2) or 6 GB (1 GB x 6) or 8 GB (2 GB x 4).
Go with 4 GB if you'll mostly just be using 1 application at a time. Go for the higher numbers if you plan on switching between multiple apps all the time.