Bicycle Mechanics - which rear derailleur?

I have a fairly old bike with Shimano Altus A20 components, and the rear derailleur broke. Can I just get a similar sized Shimano derailleur that is made for the same number of gears? I am thinking about replacing the rear derailleur with a Shimano Alivio.

that should work for you .

You can put a new rear derailleur on as long as it's for a 7/8 speed. I'm not positive, but I think there could be issues if you used a 9 speed since the pulleys and chain widths are different. If you're not using a chain with quick disconnect/power links, you'll need to replace the chain or buy replacement pins.

Here's a good deal on a rear derailleur: http://www.nashbar.com/profile.cfm?category=600083&subcategory=60001039&brand=&sku=3039&storetype=&estoreid=&pagename=Shop%20by%20Subcat%3A%20Rear%20Derailleurs

Also view the bike from the rear after attaching the rear derailleur, and check whether the frame's derailleur mount is out of alignment. If it is, you want to get it aligned so your new derailleur doesn't get wrecked (hooked in the spokes when shifting to low gear, for example).

Aligning the derailleur hanger can be done by your LBS the "proper" way, or you can usually do a reasonable job with some careful use of a large adjustable wrench. If the hanger is significantly damaged, thread the bolt from the old derailleur back into the hole before you start aligning the hanger, so the bolt keeps the hole from getting too deformed to install your new derailleur into it.

Disclaimer: you can ruin your frame if it does not have a removable/replaceable derailleur hanger and you're unlucky or uncareful. So if you're not sure on this task, hand it off to the LBS.

You can put a new rear derailleur on as long as it's for a 7/8 speed. I'm not positive, but I think there could be issues if you used a 9 speed since the pulleys and chain widths are different.

Whether the rear derailleur is designated seven speed, eight speed, nine speed doesn't matter, it will work. But you're right about the chain, to run nine speeds would require the narrower nine speed chain-

I have a fairly old bike with Shimano Altus A20 components, and the rear derailleur broke. Can I just get a similar sized Shimano derailleur that is made for the same number of gears? I am thinking about replacing the rear derailleur with a Shimano Alivio.

Replacing an Altus with an Alivio is no problem. They are 100% compatible.

Replacing an Altus with an Alivio is no problem. They are 100% compatible.

+1. Go for the Nashbar deal- for $14.99 you can't go wrong.