Firstly, what do you want to do with the laptop? Just basic work or gaming etc etc?
1) The i3 and i5 are a bit faster than the older Core 2 Duos, but the difference isn't massive. A 2.8ghz+ Core 2 Duo could probably keep pace with most of the i3s and i5s.
2)The i5 will nearly always be faster than the i3. They generally have a higher clockspeed and have certain performance boosting features that the i3s lack(turboboost for instance), which makes the i5 quicker.
3)I wouldn't worry about CPU cache, but as a general rule, the more the merrier!
4)It isn't worth purposely hunting for DDR3 laptops over DDR2, the perfomance difference is usually very small indeed. Besides, all Intel i3 and i5s will come with DDR3 anyway.
5)You're not likely to notice the difference between 3gb and 4gb RAM. Even 2gb is enough for most activities- although again, more the merrier!
6) Normally 17" laptops are designed as 'desktop replacements' and aren't for carrying around. They therefore tend to have a much shorter battery life than laptops with smaller screens.
7) Wireless N is useful if you can get it (although you'd need a wireless N router to get the full benefit). If you buy direct from Dell you should be able to upgrade to wireless N, although you may have to ring them. If you can't get it, it's no great loss as most things run fine a G speed anyway (internet etc).
Hope that helps!