If you upgrade your Subs you need to invest in an amp. As it is now, any aftermarket sub is going to be under powered by the factory amp. The 2x40W channels are perfect for the factory subs, but providing 40W to a 250W woofer, for example, is just risking damage to the cones. So a decent monoblock amp will do the trick for just the subs if you were to replace them.
The Amp that Crutchfield recommended is based on what you want, but personally I didn't like the looks of any of those bluetooth amps.
I went with the DB+ series speakers (6502s up front, and 652s in the back). Then I got two of their 8" DVC Subs. I'm still in the selection process for Amps.
What most folks I've talked to have told me is that I shouldn't waste my money on a DSP unless I am super serious into audio. A good amp has filtering, and the power will provide the clarity you want. You can always add a DSP later with a good amp set-up.
Most will be satisfied with speaker upgrades and a reasonable amp to complement it without breaking the bank, which is what they recommended. If you go for subs, though.... Like I said, get a proper amp for them. Otherwise, you'll be posting a "why did my subs blow" thread later on.
Crutchfield's website has a ridiculous amount of information to help you make up your mind.