That's difficult. The OS can use 4 GB. The problem is Vista maps sysem driver memory including the video card memory into your 4 GB limit. You can easily have 2 GB max usable RAM with some setups. Performance would suck if they mapped it out, but you lose RAM if they don't. Screwed either way. I would say go 64-bit, but Vista didn't do too well with the 32-bit compatibility in it.
You are speaking to my situation. I have 4gb ram, and with PAE enabled it does show 4gb in system properties in Vista (not so under XP) but it evidently does not use it. As a result, i am often down to single digits in free ram, as i usually run 3 or 4 browsers (incldg. 2 instances of FF simultaneously) while working on a research project, plus some other heavy apps.
The mobo itself has a max of 4gb (and it’s supposed to with 1gb sticks, but i have two of those and one 2gb stick). I wonder if Linux can handle all 4gb? Vista is slow, even after a clean install.