Thanks celliott. I didn't really get anywhere. I think there is an issue with the controller/drivers for that controller (VIA 6421A) - tried loads of distros - OpenSolaris, ubuntu, CentOS, fedora - all had problems . Rather than buying a SiI controller card (looking at £15), I'm saving up for an Atom 330 board, or similar. It will have gigabit too which is great as I've got a 16 port gigabit dell powerconnect 2716 under my desk and eveyone else in the house can serve wirelessly.
The moment I turn off the SATA drives, the system works flawlessly. I'm going to head back to CentOS and wait until I've re configured the system. It's quite simple as you say - now it's just about getting the hardware right. I'm hoping Intel chipsets are supported well (ICH7).
I really like OpenSolaris' gnome desktop but it's a very resource heavy distro. I can't use it with 512MB RAM (idles about 550MB) whereas all the others will only use 80-200MB - CentOS using only 80MB idling). Still, when/if I get an Atom board, I will probably use a 2GB stick.