You can't just compare "power ratings". Power ratings on amplifiers are subject to all sorts of conditions. Make sure you compare RMS ratings at comparable THD. 100W doesn't do you any good if all you're outputting is a square wave: it'll be loud, but it'll sound awful.
That said, being more "mass market", I suspect car amps do offer more power for your buck, even at reasonable distortion ratings.
Even a mere 1W RMS is surprisingly loud. 40W RMS from a decent set of mid-range speakers can get almost unbearably loud at close distances in enclosed spaces.
On a device like this, look for a device with good power supply filtering. Power rails inside a PC are pretty noisy, and you don't want all that noise coupling onto your audio outputs. I'd also avoid ones which are powered from the slot they sit in; instead go for devices with dedicated power connectors. Pulling much more than 30-50W off a PCI bus connector is iffy business, and the power is likely to be even noisier than the lines coming directly from the power supply.