There is known to be some performance improvement in 2.6, especially about multiprocessor (but on uniprocessor it should be somewhat better as well). I suppose it depends on what the box is used for. There is some nice features in 2.6 (bridging, brouting and some more iptables nice things, virtual server, ipsec built-in) but it you don't plan to use any I suppose you don't really need 2.6, unless your are looking for something geek to do.
I suppose you should at least update to the last 2.4.X kernel, in any case
