Well, to wind this up, I thought I'd mention what I finally went for:
Mobo Abit IP35 Pro
Intel Core Duo Wolfdale 3 GHz
4G Crucial ram
Zerotherm Butterfly cpu cooler
ATI Radeon 3870 512M
+ other bits and pieces
Put it all together Friday and yesterday and booted up ... wow!
I reinstalled SH3 up to GWX2.0 (haven't yet added mods in) and started it up to see how it is. The comp doesn't even break into mild condensation, never mind a sweat.
My thanks again to everyone for all the comments and advice.
