Okay I stuck it in last night. Just an installation note, it takes a while for the MPC to boot up when you first install the card adapter and the card. Don't panic. After you format the card, boot times return to normal.
I bought a 32gb SDHC card, and that basically went without a hitch, except that when using the USB interface, the card was noticeably slower than the hard drive I had in there. I didn't test out the internal loading and saving speeds (mainly because with the USB interface so slow, the upgrade would be useless to me) but I was able to save a sample to the card and reload it.
Fortunately I had a spare 8gb SD (not HC) card laying around, so I stuck that in, formatted it, and tested out the USB interface. Same exact speed as the hard drive when transferring samples to and from the computer. (Which confirms that its the 1k's USB interface that's the bottleneck.) Anyhow, I think I'm gonna go with that for the time being.
Just in case you aren't sure what to buy, two suggestions: avoid SDHC and make sure the adapter you buy meets the requirements stated in the sticky thread above. (It must be BOOTABLE, have NO DRIVERS NEEDED, and it MUST HAVE ALL 44 PINS and it can NOT REQUIRE A POWER ADAPTER). This is a link to what I bought:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... MEWNX%3AIT)