once you have mac, you never go back!
umm i dont really like pixen, try and acquire photoshop from somewhere - its great for doing other things besides pixel-art.
i would say get adium for your msn/aim program, works very well plus you can customise the design layout so it looks nice. You might also need WMP for mac for some video files, plus quicktime pro. Handbrake is a great program for ripping full-length movie to very high quality MP4, i just rent, burn, then take them back :p
Once you use mac for a while you really see the benfits over windows.
What mac have you got?