Lenovo will probably call the new models T420. They did just announce the X120e with AMD fusion platform.
As long as the memory module meets or exceeds the spec, then it should work. ThinkPads use Intel's recommended settings so they will take almost all modules.
You will not void warranty if you install after market memory, or add/replace parts that Lenovo has identified as customer replaceable in the hardware maintenance manual. Should your system malfunction, it is a good idea to remove the aftermarket upgrades before you troubleshoot or send system in for service.
There is an oversupply of DDR3 chips on the market, so the prices have been dropping like a rock over the last week. You can find 4 gigabyte modules for about $36 each on Slickdeals now.