Extending rpphoto's reply to be more specific:
In the smart playlist dialogue box, change the pop-up menus and text in the rule field to say "Date Added is in the last 2 weeks", then click OK and save it with whatever name you want.
But yes, play around with smart playlists, they're cool, you can add more rules by clicking the + sign, and limit to a number of songs (sorted by various values or random) by changing the right options.