New PC (and 64 vs 32 question)

[Erez]

pics to come later on..
its not a new pc as much as a very very serious upgrade
didn't go all out with this one, as i didn't was to spend too much,
but wanted it to be a PC i could live with for the next 2 year or so, while i'm
busy being a poor uni student (unlike you all i'm not eligible for a student load or what ever, so i'm paying my tuition while i'm in the uni..)
so here is what i took from the old pc and moved into the new one:
1GB ram (3200 512X2)
WD 120GB SATA
LG DVD(+-)RW
Old chassis

new parts:
Mobo: Asus A8N-SLI
HDD: WD 80GB SATA (gave my sis the old 60GB ata)
PSU: Enermax LIBERTY 400W (500W is 50% more over here.. and i wont be using SLI in the near future..)
CPU: AMD Athlon 64 3700
CPU Cooler: Scythe NINJA PLUS
Graphic Card: Leadtek 256MB 7900GT Extreme ( )
and thats about it i think..
all the parts are here, and running, other then the PSU, i'll pick that up tomorrow..

as for my windows question..
i'm now running WinXP Pro SP2, what the difference to the 64 bit?
i know nothing about the 64bit.. nothing.. i could use some info and help..
i know vista is coming up, but still..
thanks for the help guys
and comments are welcomed

EDIT- picked up and installed the PSU today and i think i'm in love