Most certificates, includsing PositiveSSL, GeoTrust, GoDaddy and RapidSSL will encrypt automatically at the highest the browser and server are capable of. If, for example someone has an older browser the encryption may be 128-bit or for an upto date cPanel server and Firefox 1.5 it will be 256-bit.
I would say that for small companies the best SSL certificates are probably RapidSSL. I have used many others with problems. I recommend NameCheap's $15.99 offer. RegisterFly sells the same cert for $13.99 though they managed to mess it up for me and EV1 servers sell them for $14.99, haven't tried them.