It loves a lot of power, a 5W is nice, but I still had issues with stuff like my wireless keyboard/mouse until I jumped to a 6W Power supply. If you are thinking about something like a usb wifi card, you pretty much have to get a powered hub.
Also consider getting some spare sd cards, Being able to switch distros is pretty nice. I'm currently switch between raspbmc and Raspberry Pi Fedora Remix depending on what I need to do. Debian is pretty good and one of the better light weight options, but I just can't remember package names for apt-get.
Also Lady Ada's Tutorials are pretty good http://learn.adafruit.com/category/learn-raspberry-pi
if you want to do anything really cool you may want to grab an adc like the MCP3008