i have windows 7 ultimate x86
8gb ram is instaled but 3.5gb is usable
x86 is 32 bit, which can only use a maximum of 4GB of RAM. To use more RAM you need to install 64 bit Windows 7.
32-bit and 64-bit Windows frequently asked questions:
http://windows.microsoft.com/en-US/windows7/32-bit-and-64-bit-Windows-frequently-asked-questions
Windows 7 64-bit Support:
http://www.microsoft.com/windows/windows-7/features/64-bit-support.aspx
To Switch Bit Type
You'll need to use the correct bit install disk - 32 and 64bit come on a separate DVD. If you bought the retail copy of Windows 7 you should have gotten both 32 and 64bit disks.
OEM includes one bit type, if needed contact the OEM computer manufacturer and ask them to send you an install disk for your computer - they may or may not do it. If they agree to send you one they may charge a small fee.
You must completely reinstall Windows when switching bit types (32/64 bit). You'll have to backup your files and reinstall all your programs.
Upgrading or simply switching between Windows 7 32/64 bit is not possible.
Make sure 32/64 bit drivers are available for your computer.
The Windows 7 key will work with both 32 and 64 bit, but only one can be installed at a time.
If you want both 32 and 64 bit installed at the same time you will need two product keys.