Slow. PC3200 DDR RAM is 200Mhz * 64 bits * 2 (DDR), for 3200 megabytes per second (that is, 3.2GBps; 800Mbps is only 0.1GBps). Dual-channel RAM doubles that again. I-RAM is, IIRC, using single-channel PC3200 RAM (or maybe PC2100), but in any case it's still way faster.
A hard drive can...