sorry if this is the only thing I can say, but I've heard that kinect is possible for sfm motion capture, Mcvee uses that

So... what should I buy to motion-capture? (I know it's REALLY expensive)

You need two depth sensors like Kinect 2 or something like that and ipi soft that is a program which allows you to record you movements and import them in SFM/C4D/Maya etc.

Ipi soft: http://ipisoft.com

I have never had an XBox, so... can the Kinect thing be connected to a computer? Or... how does it works?

you can use a camera but dont work like a kinect :\

There is Kinect for Windows.

http://www.microsoft.com/en-us/kinectforwindows/

It IS posible to use a normal webcam? I think not. Also, thank you Clinter for the link

You need at least 2 webcams. Even so, the mocap would be really inaccurate. Recommended would be at least 4.

You really shouldn't look too much into mocap unless you plan on making a studio or you have a lot of fans already and you're experienced in 3D character animation. Mocap is never entirely accurate; quirks will occur, and it will require a bit of experience fixing them up. Only a minor portion of scenes can be mocapped, unless you have the right equipment, which can often be very expensive. Finally, mocap is difficult to set up and use in general, and still considered quite experimental, so it really isn't recommended.

Well, I don't consider myself an expert animator... so ok, no mocap.

I just recently bought a couple of used kinects and tried the ipisoft trail. Overall it works fairly well. My room is not that big, so I can't move around a lot which is the biggest problem. But it does a pretty good job a tracking your motions. The captured motion does get buggered up often in the arms, though, but it can usually be fixed in ipisoft studio.