You will need 64 bit Vista Business, Enterprise or Ultimate to use 24GB
Asus P6T at the least but you could go higher if you like.
As for ram...well for 24GB you need 6x4GB sticks...4GB DDR3 sticks costs a lot this is a kit for just 12GB (3 x 4GB)....
For HDD not to sure what your looking for...
Seagate® Barracuda® ES.2 = for reliability
Seagate® Barracuda® 7200.12 = for performance/good value
WD Caviar Blue = for performance/good value
WD Caviar Black = for better performance
WD RE4, WD RE3 = for reliability
WD VelociRaptor = for speed
Then theirs the SSD (solid-state drive) to look in to at a cost and storage.