Helpful tip: you can upgrade Outlook 2007's performance from brutal rape to unpleasantly slow by disabling the Windows Search service. Windows Search feels that it's necessary to write about two gigs to my hard drive every time I open my email, and it ends up taking way too long to just check my mail. Disabling it makes Outlook run much more nicely. Searches end up taking a few extra seconds, but I almost never have a reason to search my email, and certainly not frequently enough to justify ten minutes of hard drive thrashing every day.
I don't understand why Windows Search sucks so much. And yet... out of the three products in the genre I've tried (this, Google Desktop Search, and Lookout), it's still the best. :(
(Side note: my complaints are only with POP3/IMAP access; when connected to an Exchange server, Outlook's performance seems curiously fine.)