I am planning to upgrade my PC to Athlon64.
If I install Fedoaro into Athlon64 system, which version of Fedaro should I install??
32bit or 64bit?? is I386 32bit version??

i386 is 32bit, all x86 arch's are 32bit.
x86_64 is 64 bit.

you can use either 32but or 64bit with AMD64.
HOWEVER..... 2 things to take into account.....

64bit java does not yet have a mozilla pluggin.
and there is no 64bit Flash pluggin yet.

so if you use 64bit Linux, and you want java and Flash/Shockwave in firefox, you wll need to use 32bit Firefox, which requires the installation of a few 32bit compatability packages.

in other words.... you can use either, it doesnt really matter too much at the moment.

still no 64bit java pluggin mozilla / firefox avialable??
no 64bit Flash pluggin??

ooh..then I wont go for 64bit ...

It seem there is no point for me to upgrade my PC to 64bit....>_<

i am currently running 32bit firefox with java and flash on my 64bit Gentoo.

There are advantages to amd64 arch.
for example, the AMD Athlon64 has an AGP controller built onto the processor die, instead of the motherboard.

when i first built gentoo, i didnt know it existed, and so didnt compile a driver for it.
when i discovered it, and loaded the driver amd64-agp
my games frame rate went up by 25 frames.

Yeah, even if you use it in 32-bit mode the amd64 cpus have some advantages over the p4. Still, it IS best used in 64-bit mode with the right packages to run any 32-bit programs you might need.

I think you no need to worry about it.

The all 32bit software I meet, ever now, could be well run on 64bit Linux.

Linux supported 64bit program is better than Win XP 64bit version.