What memory card and speed should be used for maximum efficiency of my new Canon A620 digital camera?

I would recommend Sandisk Ultra II or Extreme III, which are very fast cards (it's a very reliable brand with a lifetime warranty).
1-2GB is good.

50 x SD card...fast speed. 2 GB or whatever you can afford

I agree with Petra, but will add that I see a rebate on the Sandisk Extreme III cards that actually makes them cheaper than the Ultra II's, so there is no reason not to buy the faster card. The rebate offer ends 12-31-06, though.

If you don't find this elsewhere, I saw it at http://www.bhphotovideo.com