You haven't mentioned what type of calls you will be making/receiving. If you are getting/making calls from German numbers, getting a cheap German pay-as-you-go SIM (I like Smobil) is probably your best deal. Otherwise, look at the posts above.

As for the phone itself, if you have 7+ days until you go, also check out ebay for a cheap unlocked GSM phone (with quad-band). The unlocked phone can be used with any provider's SIM card, provided they operate on GSM. You should be able to get one for <$50.