Are you doing a 16:9 project? If so, you should probably scan at a 16:9 aspect ratio (cropping, etc. as you see fit).
Long story short: DPI doesn't matter to Studio. All Studio cares about are the dimensions of the scanned photo in pixels -- if you're doing a 4:3 project, then you should scan at a 4:3 ratio (or you'll end up having to do further zooming/etc. in Studio to get rid of borders, etc.). Same-same for 16:9.
And 640x480 isn't necessarily the "best" ratio, anyway -- 720x540 probably looks a *little* better than 640x480 in a 4:3 project. But even 1600x1200 will "work" and will allow you to do a little pan-n-zooming without getting TOO bad (you get much bigger than that, though, and too many photos can start to bog Studio down somewhat). Same-same in 16:9 -- 1600x900 will "work".
And as for your last question: Yes, they will show differently on the TV screen -- keep that in mind when you're croppping and placing your photos. TV screens have a so-called "overscan" to them -- there is an area (a "border" if you will) around the outside edge of your picture that you CAN see when editing in Studio which you WON'T be able to see when you play your video back on your TV set. In other words, if you tweak your photo so a person's head is *just* under the top of the screen in Studio, when you play back that video on your TV, the top of that person's head will be "chopped off".
BittMann