I generally recommend getting things as you need them, instead of just getting lots of things you might not really need. The only essentials the average person will need are a browser (Firefox, Safari, etc), a music player (iTunes), a video player (VLC), an office suite (Open Office, MS Office, etc), and perhaps a chat client (Skype, Adium, etc). Beyond the basic stuff, I'd just look for recommendations whenever I needed a program for a specific task...or google it myself. :^) Otherwise, installing junk you don't need will have pretty similar effects on your Mac as they would on your PC.