Thanks Milo, appreciate your thoughts.
The mobo manual offers CPU options including Pentium D / Pentium 4/ Celeron D processors. (Asus P5GC-MX/1333).
There are still a few outlets offering both E3400 and E5700 down here (Oz) so I'll weigh that up also - there's not a lot a difference in the prices.
I hope to keep the machine for a little longer - it's used primarily for amateur radio work. Of course when XP is no longer supported, then things will have to be re-considered. (Not sure how far away that is).
Thanks again,
FoR