|Still need to know the model number.|
If the system worked with the new RAM "in the shop", something may have come lose in transport to your house. Open the case, check all the connections & make sure the RAM & all cards are fully seated in their slots. Then disconnect all external devices except for the monitor & keyboard. If the keyboard is USB, switch over to PS/2...either use an adapter or a different keyboard. Attempt to boot & see if you can access the BIOS.