dhenke57 :
what is the os has to do wit going past max ram i can run
Its not Windows 10 in of itself, but that it is a 64 bit OS (x64). I assume you didn't have the x64 version of whatever you were using before but most likely an x64 version of that OS existed but you may not have had it installed, going by your question and the hardware you are speaking of. At the time DDR2 was common, 32 bit OSs were the norm as large capacity sticks were very expensive. Very basically that means is it can access more memory due to how memory addressing works in computers.
It is extremely rare to find 4GB sticks of DDR2 memory for a desktop system and if you do, expect to pay a lot for them - and I do mean a lot. Im talking $500 or more per 4GB stick of DDR2 800. The only reason I learned of the insane prices of DDR2 RAM is because I once wanted to upgrade my old system from 4GB to 8GB and found out.
However you *might* be able to find 4GB kits of DDR2 in the form of 2x 2GB but even those are becoming quite rare. So if you have four RAM slots you may get lucky. If you only have two slots, it is much cheaper to build a brand new PC than to buy RAM for the old one.