I believe basic dsl speeds are 768 download and 384 upload.
This should be enough speed if you are not doing a lot of internet stuff like file sharing, which takes most of your upload bandwidth.
If you are using your dsl connection for ooma and surfing you should be ok. ooma uses about 48k upload and download speed when connected.
However there are other varibles that effect your voip connection, like the quality of your connection over the internet. Do a speed/voip test at
http://www.whichvoip.com/voip/speed_test/ppspeed.html and see what your quality of service upload/download is.
This test will give you some idea what to expect.
Wayne