I don't think twin celeron-D is reasonable.

 

The dual-proc compatibility is removed from standard P4s, and I'd expect Celeries also.

 

The old Socket 370 ones were very different.

 

The only real cheap MP option today is hacking Athlon XPs or possibly even Durons (I think it involves rigging a bridge like with the overclocking) and finding the few dual-Socket-A boards.