My pc wont boot up, led's 3&4 flash, I have renewed the RAM and the fault is still there, does anyone have a clue what the problem is?

Allan Cairnduff,
Below is the information for the 3,4 diag error.
Memory modules are detected, but a memory failure has occurred.

• If two or more memory modules are installed, remove the modules (see your service manual), then reinstall one module (see your service manual and restart the computer. If the computer starts normally, continue to install additional memory modules (one at a time) until you have identified a faulty module or reinstalled all modules without error.
• If available, install working memory of the same type into your computer.
• The memory specs for this computer are listed below.
• Type DDR3 Synch DRAM Non-ECC Memory Memory Speed 1333MHz Memory Connectors Four (MT, DT, SFF), Two (USFF) Memory capacities Up to 4GB Minimum Memory 1GB Maximum Memory 16GB(with 1333MHz speed memory)/8GB for uSFF only Memory
  
  The OptiPlex 790 system supports up to 16 GB of memory in the mini-tower (MT), desktop (DT), and small form factor (SFF) chassis with 64-bit operating system. The maximum memory supported in the ultra-small form factor (USFF) is 8GB. The memory speed supported is 1333MHz DDR3 memory.
  However, 32-bit operating systems can only use maximum 4GB memory address space. Certain components within the computer require address space in the 4GB range, any address space reserved for these components cannot be used by computer memory the amount of memory available to the operating system is less than 4GB.
  NOTE:
  Memory modules should be installed in pairs of matched memory size, speed, and technology. If the memory modules are not installed in matched pairs, the computer will continue to operate, but with a slight reduction in performance.
  NOTE:
  The total amount of available memory will be less than 4GB. The amount less depends on the actual system configuration. To fully utilize 4GB or more of memory requires a 64-bit enabled processor and 64-bit operating system Thank You,
• Dell-Jesse L