MMC where the predecessor to SD and in my experience are quite slow. SDHC are the high capacity version of SD cards with capacities of 4GB+ from memory. Personally I'd go for SD cards for now, they've been around for a good while so the technology is tried and tested and good ones like the 2GB Sandisk Extreme III cards can be had for £18 inc P&P and are, in my experience, pretty much bulletproof