1. Think of Macs like any other retail PC (i.e. Dell, Sony, etc.). All of them have proprietary motherboards with severely limited user access to the BIOS (or in the case of a Mac, an EFI). So just like you can't overclock other off-the-shelf computers, you can't overclock a Mac. Unless of course you build your own, but that's a whole other can o' worms.
2. A Mac is no different than any other PC when it comes to hardware. So you have the same options for connecting to the Internet with a Mac that you would with any other PC: Ethernet, wireless, etc. Though it should be noted that the Macbook line does not offer an Expresscard slot, but the higher end Macbook Pro does.
yea, all u need is connect ur internet source (most likely comcast) to an airport (or almost any other wireless adapter) and ur click the little airport icon at the top of the screen and select the network you want.