I just received my 8GB Class 4 SDHC Patriot card today and did run some quick speed test: I just copied a 327MB file from PC to the card mounted on N800, and back to PC.

The write speed was 3119kB/sec.

The read speed was 12596kB/sec.

These were taken with a stopwatch so some error margin is expected...

It looks like the Class 4 is being utilized with N800. Luckily I did not go and get the cheapest Class 2 card.