Yes it's packaging and voltage.
1 Volt is one way to look at it, 40% is another.
Standard SDRAM and DDR SDRAM use TSOP packaging and run at 3.3 or 2.5v,
DDR-II uses BGA or TSOP-II packaging and will require "only" 1.8v.
DDR-II is backward compatible with DDR, allowing the use of DDR modules in DDR-II motherboards.
That 'wow one whole volt' allowed for
PC4800 DDR 600 300 MHz 4800 MB/s
PC5300 DDR 666 333 MHz 5333 MB/s
PC6400 DDR 800 400 MHz 6400 MB/s
It's doubtfull that DDR400 will be used for AMD Toledo:
http://www.c627627.com/AMD/AthlonXP/
They'll have to use one of these:
PC4200 DDR 533 266 MHz 4267 MB/s
PC4800 DDR 600 300 MHz 4800 MB/s
PC5300 DDR 666 333 MHz 5333 MB/s
to scale it...