How much ram does minecraft need for no lag?

There's no reason to be running a PC - gaming or not - with less than 8GB of RAM.

It's cheap and the best way to boost your performance.

Computers will always "lag" and depending on your operating system there's a limit to the max amount of RAM you can have. Commonly it max at about 16 to 20gbs, for higher end it could be 32

2gb is enough but also depends your video memory which i would recommend above 360 mb.

2gb (Windows XP) 4gb (Vista and higher) should be perfect for Minecraft. Also keep in mind processors and video cards can help a lot with fps boost.