64 bit - How to enable 4GB in my Windows 7 64 bit?

I have the following machine configuration:

• Intel Core 2 Duo E6300


• 1 x 2048 MBytes memory chip


• 2 x 1024 MBytes memory chip


• 1 x Radeon HD 4870


• Motherboard : Gigabyte 945P-S3


• Windows 7 RTM 64-bit

In both Windows and the BIOS I have 3.25GB memory available. How can I enable it to support 4GB?

EDIT: I've sort of upgraded the CMOS version, and Windows acknowledges I have 4GB but is only using 3.25GB.

I guess it is hardware after all?