I think your problem is power supply related - double check all connections to motherboard especially power and reset switches.
Have a look at 3000rpm.com - under systems and upgrade bundles. I know this shop is in Dundee but I am sure there will be a similar shop local to you. If you buy a bundle it will all work together!
You don't mention what OS you are using but if it is a 323bit OS there is no point in going above 3GB RAM as the system cannot utilise it.