Even with a nice new amp and sub, you'll still have a major lacking in midbass upfront. You should definatley look for a 6 to 8" midbass speaker in the front, especially larger if you have no sub.
You don't want to go from a 4" speaker in the front, to the next biggest speaker being a 12" or 10" sub in the rear; it's just too big of a jump in size, you'll lose a decent section of audible frequencies and won't have the decent, directional midbass in front.