With the stats Aj posted, it looks like IE5/6 deserve the bulk of testing (sigh).
NS4 also needs support. I use Mozilla almost exclusively; objectively I'd say Mozilla/NS6 should be tested if you have the time/resources. As a matter of principle, Mozilla is perhaps the most standards-compliant browser in existence, so you _should_ test on it.
In about 6 months or so, I'd stop testing for NS4. Although I was a longtime user of NS4, web developers really hate it to develop for it. We really need rapid adoption of Gecko browsers (i.e. Mozilla/NS6) and to kill the old beast (NS4).
Gecko will be embedded in the next version of Compuserve (and potentially AOL down the road), so it'll pick up at least some hundreds of thousands of users when the CServe client is upgraded.
The thing I hate about NS6 is that although Gecko is a marvelous browser rendering engine, NS6 is a gargantuan, sluggish suite.
Fortunately, we do have choices and can choose Mozilla (although a suite, it's not as heavyweight as NS6) or lightweight Gecko-embedded browsers such as Galeon or K-meleon.
Konquerer gets rave reviews, but it's hardly a blip on the radar. Same applies for Opera (which will always have loving fans, but small usage numbers). Again, if you have the resources, do very light testing on these alternatives.