I have an eMachines T2542 with XP OS. The motherboard died (at least enough to drag the integrated audio & video down with it - verified by switching monitors, cables, etc) and I ended up purchasing a new Lenovo with Vista OS. I'd like to get some of the data off the XP HDD, but am not sure if I can use the new machine to do it. Is it as simple as strapping the XP HDD to slave and plugging it into an expansion slot on the Lenovo? Will it show up as a second drive without conflicting the OS?

It will show up fine as a second drive. The only issue is if you had a password on the account you want to get files from. Those files won't be accessible without removing the password first.
Ex. : J Smith user account on XP has a logon password
T Doe user account on same OS has no logon password
All of T Does My Documents folder will be accessible on the new machine but none of J Smiths documents will be visible on the new machine.

To remove a password from XP you need a utility specificaly to remove paswords from XP OS. Do a Google search if you need such a program there are several out there.