How do I share internet with Windows from my Linux box?

I have a Linux (Ubuntu 12.04) PC connected to the internet with a Greenpacket WiMax USB modem. I want to share the Internet connection with another computer running Windows 7 Home Premium, connected to the Linux PC over a LAN. Is this possible? How? Is the reverse possible instead (connecting the internet to the Windows computer and sharing it with Linux)?

File system compatible with all OSes?

I use Linux and Mac OS X on a regular basis, and sometimes I have to use Windows. I need to use a flash drive on all three, and I need a filesystem that will work well on all of them. None of the ext’s work on Mac or Windows, HFS+ doesn’t work on Windows (or well on Linux), NTFS is read-only on Mac, and FAT sucks on all OSes. Is there a file system that would work reasonably well on all operating systems? I’d like it to work without drivers or additional installations, so it can be used on any computer.