memory - RAM Upgrade options for 32-bit windows machine with 2GB ram

I have a couple of years old PC used for semi-serious gaming. It has an Asus P5QC motherboard with 2x DDR2-800 1GB modules installed, running Vista 32 bit. The graphics card is external with its own RAM.
I recently ran into memory capacity problems with a new SW, so I'd like to upgrade. However I have no intention of upgrading to a 64 bit windows version.

A few questions:

1. I understand from "Increase in Available Memory expected by adding 1Gb RAM to a 3Gb Vista 32 bit system" that adding 2 GB won't give any additional benefit to adding 1 GB. Is that generally true, or is specific to the HW mentioned in that question?


2. Suppose I want 3GB total. The motherboard has 4xDDR2 slots. Can I add 1xDDR2 module and end up with an odd number of modules? I recall some times when memory modules were supposed to be installed in pairs, but I see no mention of this in the motherboard documentation.


3. For extra cost I could dump the DDR2-800 RAMs I have and replace them with up to DDR3-1333, as supported by the MB. Would I be able to notice the performance benefit of say, 3 GB DDR3-1333 vs. 3 GB DDR2-800 ?

Thanks