I'd definitely advise upgrading your headunit if you plan to run such a sub and swap your fronts. I would honestly say that it would be a large mistake to do otherwise...
Let me explain a few things:
First, that is going to be a very overpowering sub/amp combo for ANY stock setup.
Second, in order to run that amp / sub off your stock setup you will have to use a line-level adapter, basically splicing into your rear speakers... It's fine for some setups (BadAngel is using this setup), however when you're putting in a $500 sub running on a decently high end amp you will lose immense amounts of output / quality.
Regarding what you initially said:
Im installing a real high-end subwoofer and I dont want my speakers to get faded out with all the bass.
The only true way to avoid this is to get a decent set of fronts and amp them... I know you mention those components below, but I'm a bit confused about a couple things that I'll get to later... When it all comes down to it, it's not worth amping mids / highs off any factory headunit... It's difficult to do properly, it's sloppy, and it defeats many of the purposes of doing so (it's inefficient.. you lose a lot of output and clarity).
I plan on leaving the rear speakers alone and getting a set of components in the front and fading most of the power up there.
Ok, I always support the idea of focusing on high quality fronts and removing / toning down the rears, however in your case it poses at least one large issue (more depending on other potential situations)...
As I mentioned earlier, in order to amp your sub you'll have to wire them in line-level with a pair of speakers... The fronts would be a bitter pain in the ass to work with (considering the amp/sub will be located in the rear), so your best bet would be the rears... Fading to the front would greatly reduce the signal to the amp in this case, and thus you would lose even more output.
I'm very confused about this question / statement... In one sense it appears you will only be running the 500/1, however you then ask about a good "power rating" for those components. Please clarify this so that I can give you a better answer...
The 500/1 is a mono amp, so you will ONLY be powering your sub off of it.
Those components you linked to (or any components really) pretty much REQUIRE a decent amplifier to run properly.
Your best bet would be to get a decent 2 channel amp (perhaps the JL 300/2 to go along with your 500/1) and amp the front pair, leaving the rears alone as you stated. But this, once again, poses the problem of the stock headunit... There is no good way to do this running mids / highs off a line-out converter...
Here are a couple recommendations:
#1 Do it right... If you plan to run that sub / amp combo, run it properly off a good headunit and amp your component fronts properly.
#2 Do it to suffice... If you really don't want to swap out your headunit and are set on that amp / sub combo, step your fronts down to decent mid-line coaxials and power them straight off your headunit with no amp. Swap out your rears as well to match the quality of your fronts, because fading won't be much of an option...
#3 Do it and be miserable (and you'll have paid substantial $$$ to be this way)... Buy components and either run them straight off your factory headunit, or amp them off a line-level running off your stock headunit... Trust me, It'll sound like crap...