FreeBSD 4.? or 5.? for a quad AMD-OPTERON 848

[dkouroun at cc.uoi.gr]

Dear AMD64 people,
I just bought a quad AMD Opteron 848 server. I am interested
in running numerically intensive programs which write large files 
(about 128MB) of scientific data on the hard disk and simulations last for 
-3 days! What I desperately need is having intel-icc doing well its OpenMP
job in FreeBSD as well it does in Linux! 

1) Having all that in mind can anybody let me know which FreeBSD to install
4.11 or 5.X such that I have as few stability problems as possible and 
at the same time performance similar or even better than Linux? 

2:) I have encountered a lot of stability problems with 5-3 RELEASE for i386 
architectures. How can I upgrade 5-3 to some stable version? 

3:) Would it be preferable to upgrade to some 6-current?

Thanks in advance!
D.K.