WinXP pro and my 4GB DDR2 RAM

Hello all,

I just bought an Acer Extensa 4620 - Core 2 duo 2.2Ghz, a 2GB DDR2 RAM and an extra 2GB of RAM in the promo... I installed the extra 2GB, but WinXP sees only 3GB while the bios sees the whole 4GB;

My enquiry is the following:
- Is my windows only using 3GB of RAM, meaning I am losing the extra 1GB or the bios recognizing the 4GB means that I am using them?

- I was thinking (wrongly maybe) to update to SP3 but just reading

http://www.windowsbbs.com/windows-xp/73524-useful-information-windows-xp-service-pack-3-a.html

makes me see it as a coming headache
or get into Vista!!

My issue is that I want to get full benefit of my 4GB of RAM.

Please advise.

Thank you

p.s. I have a new desktop with AMD sempron 2.0 and 2GB of RAM and it is much more faster than my core 2 duo 2.2Ghz/ 4GB of RAM!!!!!