Sorry to add to the "issues list," but if you're looking to upgrade to newer, lighter wheels, bear in mind that the hubs of most current offerings will have freehubs for cassettes. The cogs are attached by a completely different method, and you can't attach a freewheel to these hubs. If you're planning to keep the 6-speed freewheel, you'll need a hub with threads for the freewheel, and you may find that most of those are only marginally (if at all) better than what you have now. It's quite likely that you're better off breaking out the cone wrenches, steel wool and grease, and overhauling what you've got. If they haven't been done in a few years, it will usually make a world of difference.
EDIT: Oops, I got interrupted in mid-post, and didn't see that Bill had already raised the point. As he notes, there are 8-speed freewheels, but unless you're a weak, lightweight rider, you probably want to shy away from those. Axle breakage from their use was the supposed reason for switching to freehub construction. It's possible, however, that you could go to a newer, 8-speed freehub and cassette, and find that your older "6-speed" RD is able to cover the spread with its limit screws backed out a bit.
Last edited by GCRyder; 03-24-08 at 05:54 PM.