honestly, i cant think of anything that would use 16gigs, except servers?, idk. the way i kind of understand it is, if you have 2gigs of ram but youre only using 1gig, youre still gonna use only 1gig even if you upgrade to 4 gigs. that said, my knowledge on this is only but a fraction compared to other members, but i really doubt youll be needing 16gigs anytime soon.
i didnt suggest more than 4 gigs though, you had said that you wanted as much ram as possible with w/e mobo you were gonna buy in your first post, but 16 gigs would not fit your budget since the cheapest desktop 4gig ram module is costing $145 at newegg, so youd be looking at $580 just for ram. i did find some slightly cheaper server ram costing $94, but i have no idea whether that ram is any different than desktop ram (even though they are both ddr2), and whether it would work. due to those issues, since you seemed to be after as much ram as possible, i said that you could get two 2x2gig kits.
4 gigs should be more than enough for your needs, so i would just get 4gigs. if you find that it is not enough later on, just get 4 more, that simple