Glad it worked out!  You can also look up the error codes at the support site for your optiplex as well here:

downloads.dell.com/.../optiplex-320_user%27s%20guide_en-us.pdf

UGH!   We had it working on Saturday, but came in this morning and the problem is back.  We are now getting codes 1, 4&5

I tried to switch around the sticks of ram.  I removed one and that didn't change anything.  I switched the other and that didn't change anything.   

Then I borrowed two ram sticks from  an OptiPlex 760 and that didn't fix it either,  In fact with the OptiPlex 760 ram I got no error lights at all.    

The OptiPlex 320 had 2 bars of 256 and the 760 was 2 bars of 1GB each.   If this is too much ram for the 320 then I will try to buy some 256 ram bars, but if not I will have to do something else.

Thanks

Could I have the same problem as Thespian below?    Thespian had the same errors codes and replaced the processor and it worked.

thespian77 wrote the following post at 1 Nov 2010 2:33 PM:

I know this is an old thread but I had this exact same problem today after replacing a motherboard in a Dimension 5150. #1 Diagnostic light flashes with #'s 3 & 4 solid. No video. Motherboard POST card just looped with codes and never stopped on one.

I can't seem to find any other place that addresses this specific diagnostic light pattern but I'll tell you what worked for me. After swapping RAM, Power Supply, and removing all add-on cards the only other common piece was the processor. I had a spare socket 775 Celeron D laying around in my parts bin. I popped it  in and SHAZAM! The system POSTed and booted just as it should. I've ordered a processor identical to what my client already had and expect it to be here in a day or so. I just thought I'd share this for anyone else who experiences the same problem.

So according to the error codes its still a memory fault or error.  Honestly at this point I am all out of ideas.  It seems like your memory went bad all of a sudden somehow. 

First of all, you shouldn't compare error codes from one PC model to another, eg OptiPlex 320 and Dimension 5150. 

And you can't willy-nilly swap RAM from one PC model into another. The 320 uses PC2-5300 RAM. The 760 uses PC2-6400 which probably isn't compatible with the 320, given the result you got.

So lets start from the beginning.

1. What form factor is this 320, mini-tower, desktop or small form factor?
2. Do you use onboard video or an add-in video card?
3. After messing inside, did you reconnect the monitor to the correct video port, eg to the add-in video card port, assuming you have an add-in video card?
4. Did you test this monitor on a different PC or a different monitor on this PC?
5. When was last time you replaced the motherboard battery?

Put everything back together exactly the way it was before you went inside, and then try to boot:

1. What color is the power button and is it steady or blinking?
2. Do you see anything on the monitor?