Of course in your Bios startup it shows 6GB Ram. Because that is your Bios.
All 32 Bit OS'S can not use more than 4GB of Ram. That is why under My Computer it shows like 4GB Ram.
If you install a 64 Bit OS you can use all the ram you want.
I suggest you install Windows XP 64 Bit or Get Windows 7 64 Bit as it is the latest OS.