16GB is the max for Home Premium. $60-80 will get you an Anytime Upgrade to Professional if you want to use all 18GB.  msdn.microsoft.com/.../aa366778(v=vs.85).aspx
Recently installed 18GB's of Ram memory in my Cyberpower Gamer infinity 8800, using Windows 7 Home Premium (64 bit) and in the Control Panel, System information it recognizes the 18 GB's of memory but states that only 16 GB is useable.  Is it being limited by the Home Premium operating system or is there another cause? Cyberpower Gamer Infinity 8800CPU - i7 - 930MB - Asus P6X58D-ELiquid coolingRam - 18GB Kingston Hyper xVideo card - ATI - HD 5850HD - Intel - SSD -X25-M 80 GB       - WD   7200RPM SATA - III - 1TBWindow 7 Home Premium 64 bit   
Maximum useable memory in Windows 7 Home Premium