Problem with memory on AsRock Dual-VSTA 775

Hello

I recently bough 2x2GB DDR2 for my AsRock Dual-Vsta 775 motherboard
I know that this mobo doesn't support 4gb of ram, but I found a modded bios update that allows me to use all 4gb.

now, i have a little problem, and i will try to describe it best that I can, allthrough i`m not a native english speaker.

Before the BIOS update:

My BIOS showed me that I have 2GB on slot 1 and 2gb on slot 3, but Total memory was 2.5GB
At the initial booting screen, it appeared that I have 2.5GB

After the BIOS update:

My BIOS showed me that I have 2GB on slot 1 and 2gb on slot 3, Total memory now was 4GB (which is OK)
At the initial booting screen, it appeared that I have 3GB.

When i enter Windows, it shows me 3.00GB.

My question is.. why does BIOS shows 4GB@ total memory, but the initial booting screen only shows 3GB? Will installing Vista 64 or XP 64 show me 4GB, or this isn't a windows related problem?
I heard win xp 32 should who around 3.25, but mine shows exactly 3.00GB