That T-Mobile card should do the job.
Keep your hands off Blau and simyo and all those MVNOs on the eplus network, as it's total crap due to permanent congestion and lacking support of EDGE and HSDPA.
I think Tchibo would be the best for you - I wrote a complete guide at
Germany - Tchibo - Prepaid Wireless Internet Access
You don't need special connection software - just use the one which which comes with the T-Mobile card. All you need to do is changing the APN to "webmobil1".
Tchibo SIMs can easily be purchased at any of those 1000 Tchibo shops throughout Germany - just read the linked article.
Actually Tchibo SIMs are currently sold for € 1 during a promotion, which ends today. Also I could sell you a brandnew sealed one for the price, I got it, namely € 1.
If you are interested in getting your own 3G-adapter, have a look at
Vodafone Websession PrePaid UMTS USB-Stick zum Sonderpreis - November TT Special! - Telefon-Treff - these guys sell an unlocked Huawei E160 3G-dongle (in Vodafone-branded version) + a netlocked phone (SonyEricsson J110i) with Vodafone SIM for € 40 incl. shipping within Germany. That 3G-dongle also supports GSM and UMTS at 850 and 1900 MHz, so you could use it back in the US on AT&T's 3G network and on all American GSM-networks and it's easily debrandable by a simple firmware update, which takes one minute.