I'm not familiar with the standard speakers found in the front doors of a VZ. Without knowing their rated input power, i couldn't say how much of an improvement there would be with the addition of an amplifier. If you amplify a set of 6x9's, assuming that they are semi decent, then they will of course out perform your stock rear speakers. But a side affect, is that you will draw your sound stage towards the rear. If your ok with that, then by all means go for it, but if your planning any future upgrades besides the ones you've already mentioned , then i wouldn't bother. There's no point in putting your biggest, loudest, most expensice ect speakers, the furthest possible distance away from you.
You need to have an overall goal, for your setup. Everything you buy, has to be a part of what is needed to acheive it. If you just buy random things here and there that you can afford, in search of an improvement, then your not going to meet that goal. Philthy said it really well not long ago, your sound system is only as good as your weakest component. So if your only doing this, because you planned on something else, but now you can't afford it, then just wait, and save up,
So next time, just send the missus off on her own holiday by herself, and stay at home working on your car's audio system :smoking: