I would wait to upgrade to vista
So far the only reason I can think of to upgrade is:
Halo 2 when it comes to PC will REQUIRE vista and will NOT run on XP
(I dont have any plans to play halo 2 on PC, its been on xbox for a long time now, but if this is important to you, its a reason to upgrade)
Reasons to wait on upgrading (at least for a little while):
No new file system in Vista
Extreme DRM, things such as requireing a new LCD monitor with HDMI/HDCP/DVI support to watch retail HD content in HD res (like blu-ray movies or HD DVD movies, and perhaps others).
Higher requirements which means more OS overhead.
IE7 wont be exclusive, so you can stick with XP and still get IE7.
No longer will your power button on your desktop computer turn off your computer. It now defaults to standby, say hi to a slight increase in power costs and other issues (the national power usage is going to spike up).
Similarly with laptops, your power button will no longer hibernate, it will standby. Dont expect anymore to "turn off" a laptop and come back to it next day in full battery. And you better hope its not a p4 laptop that will be staying on after you pack it into a laptop bag, some of those get so hot they melt if they are on when you pack it away. Sure standby should reduce its heat and power, but its still going to use some.
And there's the usual issues of a new OS, best to wait until at least SP1 with a lot of MS stuff
Just my 2 cents