XPS 8500 Advice Needed.

Hi

I'm looking to buy an XPS 8500 with 8GB Ram, 2T Hard Disk and 32GB SSD.
This come preinstalled with Windows 8.

I'd be looking to add an additional 8 or 16GB Ram to this, what type of memory do I need ?

I've read the SSD is used as a cache to the Hard Disk, can I install another 128GB / 256GB SSD as the boot disk and leave the 32GB SSD as the cache to the 2T Hard Disk  ?? 

Is a new SSD treated just like a normal hard disk ? Could I install Win 7, Win 8 and Linux for triple booting ?
Could 2 of those OS's be installed onto the Hard Disk ?

Thanks for any help and advice offered.