Upgrading a factory stereo

I did a quick search, but didn't find a whole lot, so here goes nothing:

I have a 2004 Nissan Murano with the factory (non-Bose) system. I like it well enough, and it's mostly filled my needs, but I'm looking at the possibility of upgrading a few other things and would like to improve the system.

I used to be into car stereos, but that was probably a decade ago, and I was more concerned with "bump" than sound quality. I do know that I'd like to keep the factory head unit but I'd definitely like to upgrade the speakers and add a decent sub. I'm looking for something that will allow me a better quality of sound, not necessarily win an SPL contest. Also, I'd like it to keep it pretty affordable (definitely under $1000, preferably closer to $500). I don't need audiophile quality stuff, just an upgrade.

So far I've found the Infinity Kappa and Pioneer Stage 2 stuff, and both seem pretty well liked but i thought I'd poll the fine members here. I've checked some of the car audio forums, but many are either a little too technically inclined, or have very different goals.

Does anyone have any experience with these two brands or have a suggestion for something else I should look into?

Thanks in advance!

Thanks for the pointers. I'm a regular visitor to a couple of Murano specific forums, and it seems that replacing the head unit is difficult to a point of nearly impossible. The center dash is one single section with the HVAC, fuel efficiency, and head unit sharing a circuit board. Custom mods have been done to cut apart the circuit board or make adaptations to allow it to be relocated but it's a very involved project that requires a pretty big amount of fabrication. I'm up for major modification on my "fun" car, but this is a daily driver and I can't justify the risk and expense of messing with fairly complex circuitry just to update the head unit. Like I said, it doesn't sound bad and I'm not looking for studio-quality sound, I'd just like to fill out the sound and make it a little nicer. If it were newer or it was my main project car I'd definitely go all out with new fiberglass insert and modded circuitry, but I can't justify that sort of thing on this car. As much as I'd love to change out the head unit, it seems that technology has changed since I dropped a single DIN single CD player in my high school ride! The two speakers I mentioned were ones that I was lead to by going on Murano forums and checking sites (including Crutchfield) to see what would fit with minimal modifications. Almost anything is going to need some adaptation since the non-Bose Murano system doesn't have a separate amp and was not meant to house composite speakers. Those two, though, had the right size of speakers to fit without major door modifications.