Based on PC system architecture, the BIOS, H/W resource such as ACPI, onboard I/O and so on, PCI or AGP VGA cards need motherboard to address. Thus, when you install max. system memory the motherboard supports, system memory address will overlap system H/W resource and MB BIOS may report lower amount of available memory.
Due to system resource conflicts, some of PC game may not able to be fully installed nor operated. In this case, we will suggest you to update your PC system with Microsoft August HotFixMS06-040 to solve this issue. After install this patch, you should able to install and operate your software without problems. The HotFix link: http://www.microsoft.com/technet/security/bulletin/ms06-040.mspx