It could still be the memory, note the part in my post where i explained about the PCB wafer thickness. The ideal thing would be to find a friend who has different memory from you and swap his memory into your PC and vice versa, that way you can eliminate the memory as being a problem once and for all. There are a few dos based memory checkers out there, i'm sure a search either on this site or on google would find you one you could boot from a floppy and use.
Have you tried any programs like ERD 2003/4? They can be usefull tools diagnosing problems.
Try disabling any onboard sound, ethernet nic's and wireless adaptors, swap PCI cards into different slots or try starting the system without them.
Uninstall any nVidia display drivers, the 3 nVidia FX cards i have owned all had severe problems with drivers and refreshing the screen both on startup and during use.
There's lots to try yet
