32bit systems will use a total of 4gb.
However say you have a 512mb GFX card, only 3.5gb will be usable as RAM.
3.2 was a tpo, 3.5 is average as most GFX cards these days are 512mb.
@1.5gb kits, nope, not as far as im aware, never seen one.
In regards to benefit of ram balancing you'd have to ask someone that realy really knows the ins and outs of RAM. I know the basics and thats enough for me.
end of the day if you want maximum RAM for your 32bit syste get 2x2gb kit, and be aware that your going to loose a chunk dependant on your GFX card's memory