I'm afraid there aren't any major brand x86 computers that fit into a double din opening natively. If there were such things, I think these boards would be a lot less lively. Besides, if you want wifi, super fast startup time, big hard drive, super hi-res screen, it won't have a low price tag.
You can get a USB GPS unit for around 40 bucks - they are pretty widespread and you can find several at mp3car's store
For AM/FM, most people go with the Visteon/Directed HD Radio - it's a standalone unit that can be interfaced with from a PC. There are also FM-only tuners that connect directly to the USB port and there's a HQCT tuner that does AM/FM over USB, but it's pretty expensive.
Most people put a touch screen into the dash, computer in the trunk or under the seat, and wire it all together with extension cables. I'm currently working on building my own all-in-one system that fits into the standard double-din opening. The problem with that approach, is that you have 7"x7"x4" of space to fit a touchscreen, a computer and an AMP. From my own experience, I can tell you that it's pretty damn tough to do (but doable).
For screens, you have a choice between Lilliput, Xenarc, Mini Touch and several others. Since the market for those screens is pretty small, they go anywhere from $120 and up. In times of $99 HD desktop monitors, paying $120 for a tiny screen seems absurd, but that's just supply and demand, i guess
Since it sounds like you haven't really been looking into car pc's that much, I suggest you browse these boards a bit more - they are full of information and are broken up by categories you were asking about. If you decide to go with a pre-built solution (instead of building one yourself), the only option I'm aware of is Revosys X500, but it's out of stock and I don't know if they make them anymore. I have no idea about service or quality of these units, so you might have to decide that for yourself.
Hope that helps!