Windows XP 32 bit only will detect 3 GBs of ram, even with a BIOS update.
Windows XP 64 Bit SP2 will detect I belie 4
Windows XP 64 Pro will detect up to 4 GBs of RAM
Windows Vista 32 bit will detect a total of 4 GB
Windows Vista 64 bit will detect a whopping 128 GB

I've researched this when I plan to move to vista. 64 bit strings run though your RAM,(if you are using a 64-bit OS) so if you have one stick it's 64 bit RAM, meaning it will use up more ram when it runs though. If you are dual channeling you will get 128-bit which will twice as much to pass through, which is why it is called dual channeling. With Vista's intense needs, it's no wonder they allow 64-bit to take advantage of 128 GB of RAM

have you got a graphics card in.... because i have 2 gig in my computer and when i use onboard it only reads 1.93gb but when my ati graphics card is in it reads the full 2 gig

Bios.......?

I believe BIOS reserves some address space that is subtracted from that offered to Windows. It also makes sense if the onboard graphics is a maximum of 64 MB.

Well slap me silly X3

Here a Bit, There a Bit, Every where a Bit, Bit.....

I think that's from "Old Macdonald's Computer Farm"...Joking!

I have no idea how much memory 64 Bit XP will actually acknowledge, but the number that can be described by 64 places of information in binary math is 1 terrabyte.

The reason a 32 bit OS has a limit of 4 GB is that 32 places in binary math equals 4 GB. Using 33 Bits (or places) in binary numbers, Intel's EMT reaches 8 GB. (2 X 4 GB = 8 GB), the next place, right? Or to be a corny, just one little, itty bitty more.

mobo/xp doesn't recognise 4gb memory

Added another 2gb of memory to the existing 2gb only to discover xp shows it as 3.25. bios repors 4gb installed, but only 3.3 available. checked out asus support for bios update but discovered there is no update. the problem is 32bit xp wont recognise all the memory due to the way the addresses are allocated, the reason for peple having different reported memory sizes is because xp takes into account any installed hardware and deducts the memory reqirement for that device from your total, eg 2 sli g/cards and acouple of pci cards could reduce the reort to under 3gb. the link below takes you to asus with possible solutions,
http://support.asus.com/faq/faq.aspx?SLanguage=en-us