There are several differences between MIX systems and HD systems, even at 44.1/48 KHz, and even with a "small" system (one PCI card)...A MIX card has 480 MIPS. An HD card has 900 MIPS.A MIX farm has 3 DSPs with external SRAM (128 K words each), 2 DSP with external DRAM (667 K words), and 1 DSP with no external RAM. The HD card has 9 DSPs each with 256 K words of SRAM.So, you can run more plug-ins at 44.1/48 KHz on an HD card as opposed to a MIX card.On HD2 systems (or higher), you can use 128 audio tracks at 44.1/48 KHz. MIX systems (no matter how many cards) are limited Other benefits to HD systems: longer cables between the computer and the I/O peripherals (up to 100 feet), more I/O channels per card (32 for HD, 16 for MIX), more flexible I/O options/routing, etc.