Make sure the power button the cpu case is not stuck.
Take out the 128mb memory stick - it adds little if you added 2GB - it might even slow it down. Make sure the other two sticks are in the first channel - assuming you have dual channel memory - or the first slots AS SPECIFIED BY YOUR MB MANUAL. Is the memory compatible with you MB - either according to the MB or memory manufacturer?
You say you lost video when you installed new memory and the HD. Did you do both at the same time? If not - after which did the problem occur. If yes, try to get system to operate with just one update. Maybe first go back to old config and see if you can get it to operate then make changes one at a time.
When you added HD - did you copy the OS to it and make it the boot drive? PLease provide more details on the process you followed - including any special software.