I have just purchased a (dv) account from media temple and I want to begin using subversion, as currently I don't use a code repository at all.
Questions:
I would not put my svn repo on the same server that you serve your websites from. Its a good and easy way to gain redundancy.
Separate repo for each project unless you are 100% sure you are the only developer that is going to touch those projects ever. Even then having them separate would be good. If you are on multiple computers you can just pull in updates for projects you are working on and not everything.