Should i just get a new motherboard?

I had two gigs of ram and all but 512 mb died. so now my machine is runnig on 512mb of ram and has an old outdated cpu. its a compaq presario sr1850nx. sohuld i just change out the motherboard and sell the machine? I was thinking of giving it a dual core processor and since it only takes micro atx boards, something with on board graphics. that way i can hook up all my extra hard drives to it and it would be a pretty decent machine maybe worth $500? what do you think? would i bebetter off just getting new ram and having an old machine or updating? I already have a desktop that had hdmi and a 3ghz dual core processor and a tb of memory so i dont really need it?