MacBook – I have a 2009 MacBook Air, bought a year ago – will I notice much of a performance improvement if I buy the latest version (Late 2010)

@Daniel has a good idea with the switch.

However, if you don't need to share keyboards/mice, I recommend getting a monitor with multiple inputs (almost all these days) and just connecting the Lenovo to one and the MacBook Pro to the other. Switch the inputs on the monitor.
The downside of this is that, even if the monitor isn't using the Mac's input, the Mac will think it's connected. So, you may get windows popping up on a nonexistent monitor, and losing your cursor now and then.

As for features: Make sure you get something with a high contrast ratio, and as VESA mount. Even if you don't anticipate mounting it right away, you may well want to later, especially if you share it between multiple machines.

Snow Leopard, Lion or Mountain Lion. As da4 mentioned, you will need to find out the capabilities of your machine and look up the requirements of the operating systems I listed.