Hi , i know it's old post but i want to answer.
1)US15 Platform (pc with Intel Atom Z5xx series) doesn't support AHCI because there isn't a REAL Sata Port (only connector). The controller is PATA, this is why if you install a SSD
will not notice much difference. With SSD
Max file transfert speed will be 65MB\s (not above 120 or more). More info on gma500booster.blogspot
2)VGA 8MB is only preallocated ram for boot system. You don't need to change it.
GMA 500 doesn't have own ram but use shared ram, after system boot some ram will be allocated to the GPU (depends by the driver\vbios usually 128MB-256MB or dynamic allocation)