Well, you can't connect a microphone to the "line in" jack and get a usable signal level. It will be so low as to be pretty much muted. You would either have to connect to the "mic in" jack or use a mixer/preamp of some sort to get "line" level.
This issue most likely won't exist with a USB-connected microphone.
"Most "line in" connections are mic or line, with a switchable boost."
While true of most outboard mixers, i have yet to see this on a consumer computer audio interface. Annoyingly enough, both of my laptops have only a Mic input and no Line input. I'd have lots of use for a Line in, and most likely will never have a use for a Mic in.
Kayjas, i suspect that most consumer headsets probably have a very cheap microphone, something intended only for telephone-like quality. If you want full rich vocal sound then these may not be the best choice. Also, if it's not USB, you're back to the same problem that it won't connect to a Line-in jack.
Kelly,
Most computers and laptops use Realtek High Definition onboard sound.
They all have mic boost in some form, but has to be accessed from the control panel.
Without boost, it handles nominal line-in (but maybe not headphone) levels just fine.
I ended up with Turtle Beach gaming headphones, plugged into the front mic socket, front headphone socket and front USB socket. The sound quality is very good.