If you have the time to conform before import, that's best. What you should do is decide on what you want to export and make everything into that.
I can't really advise on what the best standard format is for your project. I have clients that require HD for presentations and the web, and others who are perfectly happy to dumb down to their worst footage at DV PAL level. A big thing to consider, I guess, is the amount of footage you have in each format. If you have only three HD shots and the rest is off DVD, making the whole thing HD would be a pain. If there's only a couple of 4:3 shots, then maybe worth the effort. Of course blowing up to HD you will get a loss of quality on your much smaller SD stuff.
At least we're lucky in PAL land and don't have to deal with different frame rates!