Yet Another RAM Question

I'm about to buy a mac pro (first Mac since my PowerBook 520, circa 1994!). I've read a few of the threads on RAM and there still a couple of things I'm not clear on.

To start off with, I won't be doing anything too processor-intensive... i.e. no Final Cut Pro, not even much Photoshop... Logic Pro is pretty much the biggest app I'll use. So I'm going for a quad-core, and I figure that 4GB - 6GB of RAM should be fine, & I can always get more RAM later on.

I've read about how important it is to match the RAM and fill your banks, so my questions are:

- If I just get a 1GB x 2 set from OWC or whomever, to get 4GB total, does that work just fine alongside the 1GB x 2 OEM RAM that's already in there? Or do you actually need to make sure that the four sticks in a bank are 100% exactly identical in every way/shape/form?

- Let's say I go for 6GB, which would mean buying 1GB x 4 and installing that alongside the existing 1GB x 2. I know I won't get the performance that I would have if all the slots were full, but the 6GB won't actually give me WORSE performance, for whatever strange bizarre reason, than 1GB x 4, would it?

Thanks!