I'm not a 50D user, but I'll try to answer what I know.
2 - I'm using a 60 Mb/s Sandisk Extreme CF card and I can do 720p no problem, you only need 37 Mb/s for that. I gave you the rate, you do the math on how many minutes you can stash on a 16 Gb card.
3 - You should definitely not use your laptop, any laptop has a terrible mic. It's not even going to be worth your time.
Look into Rode SmartLav for a great low cost lav mic, hook it up to your iPhone or smartphone and you're good to go.
VideoMic (Pro preferably if you can afford it) is another great, low-cost, decent quality alternative.
But you're probably going dual system anyway, so why not just go for a Zoom H4n and be done with it.
You should try to be more specific, I can't really tell if you need a shotgun mic or a lav mic or just an ambient mic.
Either way starting with a Zoom H4n is a good start.
It's a recorder and a microphone in one.
All that said, is it really worth it moving to a 50D over a 550D?
It sounds like you could be just as well of with the 550D, which is a solid camera might I add.
Devinsupertramp used it for a long time and he produced high quality stuff.