Dell Dimension 8400 - Start Up Problems After RAM Upgrade

I purchased 2gb of new Dell factory RAM to upgrade my Dimension 8400 (3 1/2 years old). I removed my old RAM (512mb each) from slots 1 and 2 and reinstalled them in slots 3 and 4. Then I installed the new RAM (1gb each) in slots 1 and 2. When I started the computer up all I heard was beeps and nothing appeared on the screen - no start up sequance - no activity. So I reinstalled the old RAM back into slots 1 and 2 and installed the new RAM in slots 3 and 4.  I restared the computer and everyting worked fine and the computer showed that I now had 3gb of ram.

Everything worked great for 1 1/2 weeks - then I go to start up the computer and I have a failure. The computer turns on and I see the first Dell start up screen (where you can push F keys) then the next screen is black with a white cursor in the upper left - no activity. I do not make it to the screen where the litte blue Dell banner is at the top of the screen for less than a half second, nor do I see the XP start up screen. The light sequence on my drives go through the CD rom, then to the DVD rom and then to the 3.5 where the light stays on(does not shut off) while I see the black screen with the blinking white cursor.

So, I reinstalled all of the RAM, old and new, in the same slots and tried starting up - no luck, the same seqence as above occuired. I did this over and over (eight times) and finally the computer went through a full startup and everything worked great. However, after the first shut down/restart the compter went right back to sequence noted above.

So, I complelerty removed the new RAM and tried to start up with just the old RAM.  This worked, the computer will start up fully and I thought I could leave it on until I figureed out the probelm. However, afater two days of leveing the computer on I got a "blue screen" and had to shut down. Here is the problem.... now everytime I shut down or restart, I need to remove and reinstall the old ram, in order for the computer to full start up the next time, otherwise I get the same "Dell screen then black creen with blinking white cursor sequsiance".  I never reinstalled the new RAM again.    

So to ay the least I need help - I'm bummed becuse my computer ran flawless before I went to upgrade the RAM.

Do I upgrade the BIOS from the A03 I'm running to the A09? From what I read, is my computer stopping short of POST?

Thank You for any help.