1. Don't stay married to the idea of two 10"s. It is a cheap way to go, but for instance, one 10" could be louder than two cheap 10"s, and take up less room.
2. Just upgrade your front components, and get subs. You don't really hear the rear speakers except for fill, so spending $100 on upgrading them isn't smart money.
3. Unless you're also planning on getting an amp for your front speakers, don't spend a bunch of money on them. They will only be as loud as they currently are, maybe a little clearer.
Personally, I say you should get:
6.5 components for the front:
A dual 2ohm sub:
and a decent four channel amp:
You would use two channels for your front speakers, and two channels bridged into the sub in series. This gets you a summed channel at 4ohms, with 300w, and two powerful front channels.
In the future, if you want more, you get a second, mono, amplifier and a matching sub. Then you run the four voice coils in series/parallel for a total of 2ohms. You would then use the extra channels on your 4 channel amp to either bridge for your front speakers, or amp your rear speakers.
That would be $510, plus you need wiring for the amp, and a box for the sub. Expect to pay about $650 for everything, and I have no idea how much it costs to have stuff installed, because I just do it myself.