RAM compatibility issue?

[Schizo]

I'm trying to upgrade a dell dimension C521. I know just enough to be dangerous, which is to say I think I should be able to do this myself but I have very little useful knowledge. I currently have 2 1gb sticks of ram and two empty slots. I bought two 2gb sticks to fill the empty slots and tried to match them as closely as possible to those existing. So now all four sticks are PC2-5300 667MHz. But when I plug them into their slots the computer fails to fully start and while the lights come on and the fan whirs it beeps at me three times (long,short,short) and the monitor seems to receive no signal whatsoever.
It makes no difference to my result what slots I plug each of the four sticks into, though the slots seem to be color coded (pair of white, pair of black); any use of the two new sticks of RAM yields three beeps and nothing further.
The question is What am I doing wrong?
Thanks in advance.

 

[PP Mguire]

Max memory is 4GB. Clear CMOS and install the 4GB set into slot 1 and 3 then try it.

 

[Schizo]

Thanks! Tried that and failed but reasoned that if there's 4gb max and 4 slots, maybe they're each just 1gb max. So I traded in the pair of 2's for a pair of 1's and those work just fine. However, when the machine booted up it told me its onboard battery voltage was low. I replaced the battery and got the same warning again. I called a local repair shop and, without actually examining my system the guy suggested maybe the motherboard was failing. Yikes! If this is true how can I confirm before I either continue with upgrades or scrap it and start fresh? (What got this ball rolling in the first place was a seeming failure of the gpu, forcing me temporarily to rely on the one mounted to the motherboard.)

 

[PP Mguire]

If you removed the battery to clear the CMOS then it's going to say that. If you put the new battery in correctly then don't worry about the error.

 

[Schizo]

Thanks!