Memory Card Speed?

I have all Sandisk memory cards (both Compactflash and SD). The newest just say "Ultra" and the ones from about a year ago say "Ultra II". What are the card speeds of these? Most are labeled but seem confusing. Cards listed below with their info.

CF Ultra II 2gb (no other info)
SD Ultra II 2gb (4 inside a circle)
SD Ultra II 4gb (15mb/s 2 inside a circle)
SD Ultra 8gb (15mb/s 4 inside a circle)

I understand (I think) when the label says 15mb/s that's the speed of the card. What is confusing is when I have two cards that say 15mb/s but have different circled numbers. I thought those circled numbers represented a speed rating also. Can anyone shed some light on this? Thanks.

The "MB/s" is a burst rate (peak write speed). The circled number, Class 2 or Class 4, is a sustained throughput rating, used only on SD cards. Class 2 is a sustained throughput of 2 MB/sec; Class 4 is sustained throughput of 4 MB/sec.