hangs when mounting from /dev/md0

[Henry]

I just put a system together and tried to install freebsd 5.4 on it  
but it wouldn't boot properly... It stalls when Mounting root from / 
dev/md0

I tried two different sticks of ram... Put in FreeBSD 4.11, booted  
fine and went to install menu... Also tried OpenBSD 3.7 & Freesbie  
1.1 (5.3) and those were fine also.

What could be causing this?

Hardware:
PENTIUM 4 1.8GHZ 400FSB
PC CHIPS M916
OCZ 512MB PC3200
ATI 4MB PCI