So basically it makes the Vectors or "3D" Run faster by reading a bunch of Commands at one time and doing the PC version of those grouped commands using MMX and SSE.
Reg Caching is Register Cashing. The Registers are 128 bit and the PC currently can only do 32/64. By cashing these and not having to read them over and over saves alot of CPU cycles making it faster. As far as I know it doesnt work yet