@Dm
Why the hell do you want to buy a new stick when you have one? Assuming that your O2 stick is Huawei-made, please read this post:
http://www.prepaidgsm.net/forum/mobi...odems-web.html
Also Congstar is quite expensive. Funnily Congstar offer the 3GB pack for only € 15/month under the brand of "ja! mobil" (sold a Rewe supermarkets) and "Penny mobil" (sold in Penny supermarkets). Just pop into a nearby Rewe or Penny supermarket, buy the ordinary "ja" or "Penny" SIM card and choose the "data tariff" while activating it online. After the SIM has been activated and you have topped up your credit, you can activate the 3GB pack as described for Congstar earlier in this thread (i.e. online in their customer portal).
For recharging Congstar, ja and Penny SIMs you may also use regular T-Mobile "Xtra" top up vouchers that are easier to get than the branded ones.