temps dont sound like an issue.. Its a hardware limitation.
Wha brand of ddr400 do you have right now? Are you running dual channel mode or just one 1gb module?
Its probably not the memory holding you back anyways.. Since you said you "upped the voltage as high as 1.8" leads me to belive that you have a chip that likes to run what its made to run at and nothing more... the best you can do is live w/ it or by a better flavor of chip meaning(search around for the week of cpu, and where it was made) as this seems to be the best way to build a tweakers system.
Hope this helps ya.
Koz
p.s. i believe on the newer chips bridge locking for multiplyer changing is extremely hard to do properly if not impossible in some cases.....