From my understanding, the DSi only has a slightly larger screen, but the resolution will NOT be higher. I don't know this for sure, but I believe this is what I heard from somewhere... you know how that goes. It is comparable as to a 17 inch monitor vs a 19 inch monitor, using the same desktop resolution setting. Does that make any sense?
Honestly, if you have a perfectly fine DS Lite, I see NO need to upgrade to a DSi at all. Unless you wanna give away your DS Lite and get a new DS, but keep in mind, if you are using a flash cart, they are currently INCOMPATIBLE as of now. Who knows if they will come up with a way to hack it or make a new flash cart, but to be on the safe side, I would keep the DS Lite. Half a centimeter of additional screen space is not enough to make me want to buy a new DS. And surely Nintendo does not expect every customer that has a DS Lite to get a DSi, so new games should still work for DS Lite.
In my opinion, it's not worth it. The new PSP 3000 also features similar upgrades, such as a bigger screen, but I read that they had been recalled due to video interlacing problems with the "new and improved" screen.