Benaulim and Colva are very close to each other, and Cavelossim is only 20mins away, so it's possible to have a look at all three in a day . My choice would be Benaulim, lovely village, and beach with lots of shacks try Little Tiger. Good retaurants are Johncy's and Pedro's both by the beach . You can walk along the beach to Colva , around 25mins , which is more built up and touristy but in an Indian way, and can be rather "grubby". But again lots of shacks and restaurants, Domnicks on the beach and Kentuckee are good. You will be pleasantly surprised at the prices charged, there is also Cafe Coffee Day in Colva for some really good coffee. Not sure of the Taxi fare but should be around 1300 rs for the whole day, could be less , time it takes around 1-1/2 hrs depending on traffic and your taxi driver.
x lassi
ps- there are photo's of all three in the Goa photo album