|Well, you said you replaced a RAM. Your machine has two slots. Are both the modules the same ones that came with the machine? One should be labeled DIMM A and the other DIMM B. The manual has a cautionary statement about installing DIMM A first. Some machines don't like it if you don't fill all the slots up starting with the lowest numbered slot. And If you had a bad RAM module, that could be the case. So, its possible one or the other is bad. Also, is it possible you installed a mismatched module?|
Compare to this (apart from size).
If you can't enter CMOS (Configurable BIOS) still, then I'd still be suspecting the RAM.
After replacing the battery, the BIOS checksum error would have appeared due to CMOS having been cleared by the battery being removed. It should automatically reload the defaults. You could force it to do that again, by removing the battery. Once you can get into CMOS, there would be an option to select factory defaults and possibly other saved configurations. I haven't ever noticed that option, but then there are many different BIOS out there.