id stick with 2 1GB sticks, ecspecially if they both have CAS2.5 4 sticks is just a hassle. plus you can dual channel anyway id think 4 might make it slower because itd just have to access more memory slots *insert intellectual computer talk* and thats how to cure world hunger...
3) It's not really likely that you'll need OVER 2GB of RAM, so I wouldn't worry about using up all the slots.
Five years ago people said we would never need more than 512KB of ram..lol I remember games like the original Rainbow 6, and its minumum requirements was 64KB of memory. Now todays games, like BF2's minumum requirement is 1GB. As graphic games become more demanding of a systems hardware, you will see these numbers continue going up.
get 2x1gb. unless you have the new dfi sli-dr expert board, it is absolutely impossible to run 1t timings with 3+ sticks of RAM, and 2t is about a 30% decrease in performace for memory. however, if you need to extremely high FSB, i would go with 2x512 since there is no 2 gig packs rated at above pc4000 right now, and they also have fatter timings.