I have been doing a lot of testing on different browsers recently at work,
IE7 - let XP update itself and then remove (that way it doesn't reinstal)
Firefox - may as well use IE7 (not a lot of security difference - I can get the figures if anyone wants to know). IE7 works better.
Opera - am using for most things (paypal doesn't like it much) and IE6 for the rest.
IE6 - not bad, shame about lack of tabs.