I wouldn't think it's a dead CMOS vattery. I've had that hapopen before and the computer will generally start up fine every time, but put you into a BIOS setup screen every time where you will have to re-enter the date, time, and IDE specs. Then you lose that when the computer shuts down. But I think it weuld still start with a dead battery.

In any case, replacing the battery if it's so low certainly cannot hurt. And putting in RAM will at least get the POST to the point where it can give you some beep codes which you can then look up in the documentation, or most can also be found at the manufacturer's website.