According to your figures the ATP Class 6 (in the internal mmcblk0 slot) is about 10% faster than the OCZ in the external/mmcblk1 slot, and both cards are about the same speed as my Class 2 8GB Transcend (~19.5s).

The only thing I can deduce from these figures is that the read performance is not affected in any way by the "Class" of the card and a Class 2 card will transfer (read) data as quickly as a Class 4 or Class 6 card, assuming all other variables are the same (high-speed and wide bus both enabled). Also, based on your figures a "fast" SD card is unlikely to be faster than a standard SDHC Class 2 card (your OCZ is slower than my Transcend).

As for testing "write" performance - this is tricky as the OS will buffer writes giving unrealistic performance figures. Hopefully someone else can suggest a methodology to test write performance.

Until we can get firm figures on write performance, if anyone is looking to buy extra storage my advice would be to buy Class 2 SDHC cards, avoid SanDisk (no high-speed support) and don't spend too much as you're unlikely to see any real benefit for the extra outlay.

I've had a good experience with Transcend - reasonably cheap 4GB and 8GB Class 2 capacities (in the UK £25 and £50 respectively), and they have high-speed and wide bus support.