Hi Phil welcome to UKY!
First, you should remove your email from your post (we don't want you to be flooded by spammers).
Second: Tier 1 Visas are no longer an option for anyone outside the UK as that visa path has been shut down since 23 December 2010. Tier 1 is only open for inside UK applicants and it will close after April 2011. After that Tier 1 will no longer exist except for the Investor & Entrepreneur categories. So unfortunately that is not an option for your girlfriend.
Her only option at the moment (for a work visa) is
Tier 2 General -- which requires her to find a employer in the UK to sponsor her visa. Be aware that there has to be no one else in the UK or the EU that could possibly do the job that they're offering to her. So it's difficult at the moment to obtain a Tier 2. And I don't believe that Acting is on the Shortage list.
As she'll be a recent graduate you can look at
BUNAC Intern in Britian program, but it would only give her 6 months in the UK and then she has to leave and cannot apply for a new visa while in the UK so no matter what at the end of the 6 months she has to go back to the US.
Unfortunately most people have come to the point that you have and make the realisation that you have to consider your relationship and what stage you're at and if you're ready to make the commitments in order to be together. This usually means marriage. There really is no visa that will allow her to live & work in the UK just so you can be together as bf & gf (we call this the 'lets shack up visa'

) it just doesn't exist.
We have a dedicated Visas section which has loads of info, feel free to nose around there and ask as many questions as you like.
Again, welcome to UKY!