It will run fine in an XP VM... assuming your host machine is reasonable.
While there have been great advances since 8.1 (which was a solid version once all updates were applied), spending $2k+ on a new version and learning all of the changes from 8.1 doesn't sound productive to me if you don't anticipate a subsequent project, and your current project is something you readily see how to do in 8.1.
While GS themselves have this Wiki page listing the new features in each version textually:
http://www.archicadwiki.com/ArchiCAD%20versions?highlight=%28versions%29
Jared Banks recently posted a link to this excellent visual list of features from Graphisoft Connect in the UK that lets you pick an earlier version (only as early as 10 though) and see everything that has been added in the current version. (Not all changes are of course listed ... 100's of small things don't make the 'big picture' lists.)
http://www.gsconnect.co.uk/calculator.php
If your current project is quite involved / detailed, then you may find that there are enough new features to justify the expense and learning time.
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