Need to know wheels and total weight of current bot.
Also, your current setup will do okay. Add two more CIMS and you'll do as good as a lot of other teams. If you don't, your acceleration will be kinda bad (it might take half the field to get up to speed). Think of motors as torque generators (with max RPM limits).
Also, search around on the forum first. There exists at least one other thread that discussed this very topic (with some more generalities), and you're not the only team with this question, nor are you the only one that needs answers. See
http://www.chiefdelphi.com/forums/search.php.