XP to XP is the common case, and I've never seen practical gigabit saturation in this configuration for Windows file transfers. FTP and other such can be a different matter, but we focus on Windows file transfers because that's a practical need for most people.
Most people actually see far less than you -- around 30 MB/s is more typical. 60-80 MB/s is seen in some cases, but even then, you can't simply generalize from that to effective gigabit saturation. There is a difference between XP and Vista for file transfers, and until you show XP effectively saturating gigabit, you're just speculating that "XP is just as fast as Vista".