Google 88.4% Yahoo 3.7% T-Online 2.3% MSN Live Search 2.0% AOL Suche 1.0%
T-Online is a local german provider, owned by the german telecom. Generally, I'd say germans use google.de, since google redirects to that if you ask for google.com from germany without any parameters.
I don't know about that since I rarely work on optimizing english pages. Our bestranking pages for competitive keywords in germany are, of course, the ones that have been around for a couple of years. We don't have a very strong incoming link base, but it seems not to hurt us. In general, I'd go for the standard on- and offpage optimization before focusing too much on specifics of google.de -- you know, there's only one rule: don't mention the war ;)