Jailbreaking is reversible. It modifies the main processors firmware image by allowing write access to the restricted filesystem. If you want to remove the jailbreak and bring the Touch back to iPod factory spec just do a clean restore from an Apple firmware image. Unlocking iPhones has been, at various time but not currently, a difficult to reverse process. But for iPod Touches, unlocking isn't necessary.
QuickPWN is long gone.

You need a tool called "redsn0w". Do a google search on "iphone dev blog", click the first hit in google, and then click on redsn0w on the left-hand side of the screen.
The Touch has, if I remember right, 128MB of RAM and then 8/16/32GB of Flash "disk" space. You have ~40MB of RAM to run apps and this can run out if you are doing things like playing music while using Safari, but it's rare. If you are concerned, there's a free app called "Free Memory" which maps your RAM and attempts to free it up.
As far as "disk" space, it's my understanding that as of OS 2.0, all apps are stored in the large partition of the OS and so you can have as many apps as you can fit in the main memory of the device - ie. 8GB. There was a pre-OS 2.0 issue with jailbreak apps loading into the OS partition. This issue doesn't exist any more.