It might run okay, and you might be able to do it, it's technically possible, etc, but these machines would be better with period-appropriate OS/2 Warp or Windows NT 4.0/2000 pro or 98/Me, and unnetworked.
If you want something to run Skype on, Apple has revved its lowest-end iPod Touch SKU to be the new fifth generation, but without a rear camera, and you'll get hours and hours of battery life on it. To be honest, at $230, you'll probably spend less than you will on all of the parts necessary to make a ThinkPad 600-series machine run XP or 7 well enough to even get Skype installed. (Let's pretend you already have an XP or 7 license, because those will cost you.)
Also, modern Ubuntu will almost certainly not run. You might be able to get the current version of debian to install, but good luck with a desktop environment that will run well on something of that age.
If you want something to run Skype on, Apple has revved its lowest-end iPod Touch SKU to be the new fifth generation, but without a rear camera, and you'll get hours and hours of battery life on it. To be honest, at $230, you'll probably spend less than you will on all of the parts necessary to make a ThinkPad 600-series machine run XP or 7 well enough to even get Skype installed. (Let's pretend you already have an XP or 7 license, because those will cost you.)
Also, modern Ubuntu will almost certainly not run. You might be able to get the current version of debian to install, but good luck with a desktop environment that will run well on something of that age.