At the moment I'm getting 5.5Mb/sec from a high-performance SD card.
mmcblk0 44.00 5632.00 0.00 5632 0
mmcblk0 43.00 5504.00 0.00 5504 0
mmcblk0 44.00 5632.00 0.00 5632 0
mmcblk0 44.00 5632.00 0.00 5632 0
That same card gets 15Mb per second in a $3 USB SD card reader connected to my PC. Also the cheap SD cards get 15Mb per second in that cheap reader, so I'm guessing that the SD card reader can do about 15Mb per second, while the high performance cards can do more.
As SD versus SDHC: SD cards cannot address more than 2G. That's why SDHC was invented. NO card larger than 4G can be SD and not HC. The raspberry pi handles SDHC just fine. I have two 2G cards, they haven't been closer than about 30cm to my 'pi. I've always used 4G or 16G cards.