Bicycle Mechanics - Replacing rear derailleur - Need help - Road bike

I have a 1996 Trek 470 (http://www.bikepedia.com/QuickBike/BikeSpecs.aspx?ItemID=52758&Type=bike) with the original derailleurs but modified gearing (39,50 front, 12-21 rear 7 speed). I just put the new rear cassette on and during adjustment, I noticed the rear derailleur is very sloppy in its mount, even with the bolt tight. I don't want to bother repairing it since it's a 13 year old Shimano RSX on which I've put 600 miles plus a crash this year alone, but what should I replace it with?

I'd like to keep it a 7 speed, but can I use a 8, 9, or 10 speed derailleur just as well on a 7 speed cassette? Since my shifters are original (shimano rsx) and I don't want to change them, should I stick with a shimano derailleur? Have their been any major changes in derailleurs that would prevent me from using a modern one on this bike?

Thanks a lot, and point me in the right direction if someone has already covered this.

"can I use a 8, 9, or 10 speed derailleur just as well on a 7 speed cassette?"

Yes, but try to find an 8 or 9 speed derailleur. A 10-speed would probably work but the cage would be on the narrow side for your wider chain. If there is a width problem you could run an 8-speed chain, maybe even a 9-speed chain. Do not buy a pre-9-speed Dura-Ace derailleur, the actuation ratio would not be compatible.

"should I stick with a shimano derailleur?"

Yes, definitely.

Al

Could I use a Shimano Ultegra RD-6600? It says it's a 9 or 10 speed.

Could I use a Shimano Ultegra RD-6600? It says it's a 9 or 10 speed.

That's a 10-speed, see my first post.

I saw that, but how do I tell what width chain a particular deraileur is made for? Do ALL 10 speed deraileurs require a smaller chain, and ALL 7-8-9 speed deraileurs use the standard width chain? Looking for one on ebay, so I need to be able to tell with minimal information.