either ur cpu might be gone or u need new mother board sounds like i had same problem last year when i upgraded bought a faulty motherboard did exactly same thing screen b;ank and then reboot or shut down blah blah , got new motherboard and cpu and fine
also try with different ram as well befor you buy new mother board could be that as well if u dont think it is graphics card, if all else fails i can ask my friend tmz but he might say one of those things also,
so......
1st , try different ram
2nd , motherboard - cpu. search on google of the type of motherboard you got and find out all the faults and do that with ur graphics card
3rd give up get everything new
oh one more thing its a 4th one you should do but do it first backup and format if u havent done it firstly