I don't think a HD would cause shutdowns... Once the OS is loaded it's in memory. If the HD is bad it will give errors, be unable to read data, unable to write, missing, nonresponsive, but the HD going bad while in-use will not shut the system down. That points to a different HD problem.
I say if you can't upgrade just get an older motherboard.
Check this out, on newegg:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157097I've got the socket775 (Intel) version of this. You can use your old ram and old video card and upgrade later.
On mine I get the option of DDR1 or DDR2, but that one only seems to have DDR1 (DDR 400).
It's only $67 USD, as well.
That should keep you going for a while.