An off the shelf office PC maybe stick and extra HD in it to up the storage and re-install the OS to clear off any crap would probably be cheaper theb buying components for an office use none gaming system - plus also store warantie.
That may well be a 939 socket processor, because it's older and people would rather upgrade then buy a new rig - the high end 939 chips cost a lot more then their AM2 counterparts - the same is tru of DDR memory, you can get some very cheap DDR2 memory where as good DDR can be 2x or 3x the price.
Personaly I'd salvage everything except the Mobo CPU and RAM - by a combo deal for the 3 and build a new one but I like building rigs. Don't rule out the off the shelf option if you're not so into it.
Also I just realized that 939 and AM2 is not the same socket, and I am not able to find 939 processors in my local store. So at the end it does not make sense to upgrade to a 3800 processor, for the high price I would pay as you say.
Same for the memory... you're unforrtunately right, as I can find 1Gb DDR-400 at 30€, while 1Gb DDR2-633 for less than half the price. MAybe I'll go for another Gb of RAM plus a new HD an keep the system running for another year before starting fith another motherboard and all the stuff inside...