OK so I guess XP wins the day.
I was avoiding PCI because of height and that 'working loose' problem, though I do have just under 4" if I keep it comfortably (.25") away from the seatbottom. I'd prefer not to build a full case, but rather just mount it to an old motherboard tray. If I did end up with a PCI sound card, I was looking at either this Auzentech or this SIIG, though I'm not familiar with either brand. They have the breakout style cabling and are low profile, the former which I'd prefer, the latter which I'd require. I was figuring that using the onboard sound would change how I purchase adapters to wire everything, and would leave me with adapters that I might not need. That may not be the case, as those breakout cables look like they are terminated with 3.5mm. So I'll be needing those 1/8"-2rca + rca y ->rca cables after all.
I didn't set a budget, but I'm trying not to break the bank as well! That being said, unless I hear cries of "you're an idiot" and "wth is wrong with you" and the like, I'm also considering going to a micro-atx board and a lower end c2d to beef up generally. Like this combo with an ECS board and an e6300 for 115 shipped after rebates and such. I already have a low profile cooler sitting around NIB. I think the performance gains would be tremendous for 35$ more, although I suppose its possible that I'd not notice it. Perhaps in web browsing flash sites, or a basic gamepad-style game or something.
I might take this route and get the DSATX and downgrade my subwoofer, as I'm only throwing out 300 watts to it anyway, not a dedicated amp. Not lookin to rattle the trunk, just hoping for some bass in my music as I was disappointed with the low end performance of my speakers when I had them in the eclipse. Perhaps a pioneer 10" for ~120$. That would bring my total down 200$ or so but possibly still give me decent sound. It would probably fit into my wheel well a bit nicer too as it can use a tiny volume box.
Anyway, my amp sold for 75 +15 shipping, I expect around 300 for my current headunit when I sell it, I have some misc electronics that I expect to get ~500 for. So thats about 900. As long as my out of pocket stays around 500, I'm peachy.
Keep 'em coming, I'm thrilled with the input!