I have a Google Analytics account with all my (en my clients) websites linked in it. This is great to manage all analytics in one place.
I'm now running a Google Adwords campaign for a client (with another email address than my google analytics account) and I want to keep track of Adwords stats in Google analytics.
Is that even possible? Or do I have to create separate google analytics accounts for every client I'm running Adwords for?
Not possible. However there's a way around that: create a Google Analytics account for your client's email (or get them to do that), link the two up, since they will be on the same account, and then get the client to give you access to their Google Analytics account (Website Profiles -> Edit -> Users with Access to Profile).
They can give you admin access so that you can set up goals etc. Once they do that, their site's statistics will appear under your Google Analytics account, and the Adwords and Analytics accounts will be linked.