Limewire is "hashing" the files (creating a unique identifier for each file). I'm no expert, but the waiting is like the "verify disk image" process that takes so long when mounting CD's or .dmg's. Don't worry--it can be very processor intensive at first, but helps others make sure that they get the exact file they wanted.

You could start by just sharing "rare" files: This would be better appreciated and save you the time hashing files most people can get anywhere. This would also save you some frustration if it turns out that your built-in OSX firewall and network connection make uploads very difficult.

File names are easily faked, and would mess up downloads from many sources, so the gnutella net has decided the time taken to create the hashes is worth the wait.

More exact technical answers are around, so post back if you need a better answer.

Short answer--share a few good/rare files until you sense the demand for more, and don't worry about memory--OSX handles it automatically.