Help me pick an amp and sub

I have a couple of points for you to consider. 

First, you may be happy with the way the factory speakers sound now powered off the factory head unit.  But this does not mean you will be happy if you add an amp to them.  Most factory systems are rated at 20 watts or less, and OEM speakers generally are designed for maximum output at very low power input.  In other words, they cannot handle higher power and will not be suitable for adding an amp.  Second, on the same topic of the factory speakers, often they are low impedence drivers (2 ohms) again to try and squeeze a bit of extra SPL out of very few watts of input.  So, if you try to connect them as you describe (4 factory speakers driven off 2 amplifier channels) you run the risk of placing too low an impedence load on your amp, and could damage it.  Also, adding an amp will show up all the deficiencies in the OEM speakers, and they will sound much closer to crap than to good with higher wattage input, even if they could handle it.

You need to decide what you are really after in your truck.  From your description, it sounds to me that what you are really trying to accomplish is to make it louder and add a sub woofer.  If this is true, then I suggest you might be happier with a limited budget to simply add the sub first.  Get a good system with a dedicated sub amp that will fit into your truck.  There are some nice stealth enclosures for your GMC which will enable you to keep as much interior space as possible.  Or, you could easily add something like a Bazooka self-powered system and stay within your budget.  Then, after you save a few more $$, make a second purchase and install new speakers to replace the factory ones, and add a 4-channel amp to drive them.  If you are serious about wanting better SQ, then you will also need to replace your factory head unit, but that could be your third step.

The Alpine amp you list is a fine choice.  I also recomend US Acoustics as good quality entry-level amps.  The USB-4065 is half the price of the Alpine.

Cheers!

Clarion and MTX make good stealth enclosures for the Suburban/Yukon  that replace the center arm rest between the from seats.  The MTX Thunderform enclosure holds a single 10".  Loaded with an MTX sub and powered, they run about $350 Canadian, I believe, which may push you over your budget unless you can find one on eBay or something.  I've installed that one before and it sounds really nice.  The Clarion comes with an 8" woofer and is a bit cheaper.

I missed the bit about using your laptop as a source.  Playing MP3's?  Make sure your sound card is the best you can get and that you have the necessary output adapters, as the sound card will not have RCA outputs.   So yes, with this you will need an amp.  I suggest you use a 4-channel amp and run the OEM speakers off it, one speaker per channel.  This way you'll stay safe.  A seperate sub amp is the way to go in any case.

Here is a wrench to get thrown into the spokes. How much are you going to pay for the eq? Why are you using the computer as a source? If it because of MP3 or storage capacity or whatever, this is a highly distracting and very dangerous way to use an audio system. It is also 100% illegal to use this in Canada according to our great motor vehicle act. If you must head down this path, so be it, do it at your own and my own risk, I hope you are not on Vancouver Island, we already have enough bonehead drivers here, no offence intended.

Post up a couple of comments and lets see if we cannot get you started on a similar but safer and better path. I don't want to nix what you want to do, just need to understand what you want to do. Make sure you post up about the dollars on that eq.

spl_enterprise: I've had Memphis recommended to me before (their 5-channel amp, which ever it is). When I checked it out though, it seemed a little pricey. I'll take another look.

forbidden:

Thanks for the info on the legalities. What exactly makes it illegal though? Is it the physical pc? LCD? Because there's lots of vehicles with LCD's, DVD's, GPS etc. BMW 7-series even run Windows.  Just looking for clarification on this, as yes, I don't want to break any laws here, or put others in danger.

I will be using the computer for MP3's, DVD's, GPS, and ODB-II analysis. I'm writing the software for the interface now so that it is customized to my needs (which is keeping my eyes on the road and listening to great tunes). There will be external buttons like a stereo for controlling the music as well, so I won't have to fumble with the screen while driving. Once I load a playlist, there will be very little driver intervention.

I'm likely going to buy the equilizer on eBay. I've seen it for $50US new.

-dyno