Patriot and ADATA have worked wonders for me, both in the players and cameras! As for specs, the class-rating simply states the number of MB/s the card is rated to provide for sustained write speeds. A higher class will mean less time spent waiting for your Rekordbox exports to complete, but you'll also pay more for higher class cards.
If you were to max out the link system with 4 players, all playing 24bit/96k files from one SD card, you would be pushing 2.25MB/s off the card, far below the read speed of even a 26X card (which is rated for a minimum of 4MB/s. Some Class 10 cards boast read speeds of 20MB/s (133X), those would have no problems with data rates.
That said, there were a couple of people who had issues with certain brands (I think it was Kingston -- sorry, couldn't quickly find the thread), but any SDHC card should do the trick -- and they're not expensive. Don't buy SDXC, they won't work.
