You could just use your old XP CD (assuming you still have it) to make a new XP partition on the new machine. (The fact that the new machine will have more than 4Gb isn't a problem -- XP will just ignore the extra memory.) But that will mean days (or even weeks) spent installing all your old programs and plugins, etc. (not to mention having to deal with dual boot issues), which doesn't seem worth it just to finish one project off. I would just use the old machine to finish your existing project off, while in the meantime building the new one ready for your future work.
If you think you might want to revisit an old project then render everything as audio so it can be opened in your new W7 version of Cubase (because you will have to get a new Cubase, 2.2 won't work with W7).
Note that the XP mode on W7 is apparently pretty limited, you won't be able to do much with that.