hello there

I have a lot of compiling errors and failures when i try to emerge openoffice, firefox, or even emerge --update world (on glibc for example)
messages are often something like :

PS : forgot to tell that something like "aclocal" is often mentionned while it's compiling, what is aclocal ? i found something about it but what i am i supposed to do with it ?

When you compiled your kernel did you perhaps enable preemption but disabled SMP? Preemption will not work without SMP, even though you only have one CPU, on PowerPC. A system with preemption but without SMP will behave exactly like you described.

aclocal is part of GNU autoconf (configure scripts and what not). Running aclocal will generate a script in the current dir which configure will use to figure out what software you have on your system etc. Normally, as in 99.999% of the cases, you don't have to run aclocal manually unless you are developing your own software using GNU autoconf.

mm i used Genkernel and haven't enabled or disabled it. How can i sort it out now ?

genkernel should set the preempt and SMP flags correctly. Just to be sure, run:
gzip -cd /proc/config.gz |grep PREEMP
It should say: CONFIG_PREEMPT=y
Then run:
gzip -cd /proc/config.gz |grep SMP
It should include: CONFIG_SMP=y

..or, you can do what Aaron said

and by the way, my system says i've got 768Mb RAM when i have in fact 2X512Mb

!!! Digest verification Failed:
!!! /usr/portage/distfiles/gcc-3.4.4.tar.bz2
!!! Reason: Failed on MD5 verification

Do you have the latest Open Firmware? I had 2 CL3 512MB sticks in my machine and when I upgraded the OF, I had to take one stick out because I had all sorts of crazy problems.

@SoundSquare:
Maybe you can find someone who is willing to trade your two 512MB sticks for one 1GB stick? AMD64 users generally want two sticks rather than one since the CPU has dual channels, it's possible you could find someone who didn't know that and accidentally bought 1*1GB (like one of my friends did)