We are upgrading our operating system to Vista and the SAP GUI to 7.10. Do we also have to upgrade our GuiXT 2006 Q1 ?

Yes. We have had a couple of GuiXT adjustments for the SAP GUI 7.10 version, after version 2006 Q1, so I suggest you install  GuiXT 2008 Q1 or above. This version will be delivered by SAP with the next SAP GUI 7.10 patch as well.