I did not tried Vista even I have it (remove the OS that shipped with the notebook and replaced it with XP). I jump from XP pro to Windows 7 64Bit recently. Everything seems to be running fine except couple issues.
The KonicaMinota Laser printer driver no longer works under 64Bit environment and the manufacturer do not have a newer driver for it and will not make one in the future.

Also, an opensource RAW converter run very slow even under XP sp2 compatible mode. Other than that, everything seems to be running fine.
As for MAC OS, since it is gaining market share, so more and more Trojans and worms are targeting it nowadays. If your machine is not infected, good. But I will still recommend scan it just for the safe side.
In April this year, a botnet that was created with MAC OS machines has been activated. In other words, there are a lot of Mac based zombies on the NET and become part of the botnet army. The owner of the machine may not notice in the beginning since some of the trojans just hidden inside the machine. But when the botnet was activated and the particular MAC machine join the attack, the user may noticed performance drop.
How much of the drop depends on the user internet pipe size, computer processing power as well as the malware itself. It was reported that some malware took 100% resources of the Mac machine.
Since botnet is a money (I learned that US$10,000 a day to attack a site and some botnet owners are multi-millionare) generating business (not legal of course), I am sure they will go after any machines that they can use for generating cash.