I only really have two things to add here:
1. Don't ignore the relative difficulty of building a 10-band parametric EQ, for Windows, that you can adjust while still paying attention to the road. I'm not aware of a sound card that has this built into its control panel, so that means you're going to need it in your audio player software. That instantly rules out a number of the best players, such as Foobar2000, because while they have all these features and more, they are far from something you could adjust while sitting at a brief red light. Also, believe me when I say that the Audigy 2 NX isn't a fair comparison to the empeg. Good luck getting 4V out from that. Expect to either add something like an AudioControl Overdrive, or skip the Audigy and get a more serious sound card. Personally, I'd budget about $600 for the carputer sound card alone.

2. My empeg car system sounds awesome, and your buddy is welcome to give it a listen if he's near North Florida.
empeg Mk2a / 3.0a8
Rainbow Profi Kick tweeters in front
Rainbow Profi Kick 6.5" front and rear
2 x Alumapro Alchemy MX 10"s
Precision Power DCX 800.5
Soundstream Renoir (fully Class A)
Audio Control DXS digital crossover network
Fully active, no passive crossover components anywhere
Some big capacitor I had lying around
Four figures worth of sound deadening from SecondSkin

It would take a very, very serious carputer setup to beat that. In my opinion, the next significant step up would require full 'room correction', a la TacT Millennium / DEQX / DRC

The empeg is a very serious piece of car audio gear, and I wouldn't decide for or against it by looking at unverified specs from various semi-trustworthy manufacturers.