How Many Mbps are Best for a Residential DSL?

The question isn't usually "how much do I need?", it is usually "how much can I get?". The only time you would really inquire about the "need" is if you have a certain purpose for the DSL and you want to make sure you have enough speed to accomplish the "need." For example, if you want to play online games like City of Heroes or Warcraft. With them you would have a need.

If you really want an idea of what you are dealing with in regard to "speed" read this page. http://compnetw  speedcompare.htm

If you are really stuck on DSL as a service read this; http://compnetw  e/f/dslspeed.htm

You can also test your DSL speed (once you have it); one of the sites to test at is http://www.dslreports.com/stest,

As Suntydt says, it really depends what you intend to do with your DSL. I have 1.5 MBPS, and have had for 3.5-4 years. I usually only use it for emails and basic web-surfing, but I've also used it to download a couple of major program upgrades, which took five minutes or so. I've also downloaded audiobooks as big as Alex Haley's "Roots." That took maybe, what, about 10 minutes? I tried the 3.0 MBPS DSL for a couple of months and couldn't tell any difference.

Reasons for getting the higher speed for your DSL would be:
Advanced gaming like Suntydt mentioned
Multiple computers
Frequent downloading or uploading of files
You won't notice much difference in speed for basic internet functions like e-mail and light browsing.

I suggest you running a speed test on the following website:
http://www.adslspeed.com