Memory requirements for Mac and PC are more or less the same. 2GB should be enough for normal use - using iLife, browsing the web, doing word processing, etc.
However Mac mini has only 2 RAM slots. If you now go for 2GB (2 pcs of 1GB) but some months later you find that you want more, you can only buy new RAM (2 pcs of 2GB) and throw away the 2 old pcs of 1GB RAM, because RAM prices drop very fast.
Also if you'd like to run Mac OS X and Windows at the same time (as you said, you are used to Windows PC) by using
Parallels Desktop or
VMware Fusion, 2GB is not good. Both systems will not be able to run smoothly.
Hope this helps!