one trick i use is to create a virgin system folder (don't delete the old one) with only the apple extensions, control panels and fonts.
then see if your system mounts DVDs
if yes, check for a confilct
if no, then check hardware
TechTool Pro has a good RAM check routine that u can literally run for days if u wish
i think there are some freebies out there but if u use ConflictCatcher, i am sure you would like TechTool Pro 3.0.9 (i believe is the latest incarnation) as one of your tools
it comes on a bootable CD
for more info.