newer p4 2.4ghz?
I'll assume its a 2.4Ghz Northwood -- old old core, before prescott even, if you can remember back that far (prescott days -- when intel meant 50C+ Idle temps and big clock speeds)
so, that said, you should never expect it to perform well in today's games, especially not in BF2142.
its surprising you get 15-25fps with such slow ram and cpu to be honest.
If you really desperately badly want to be able to run that game at a playable fps rate, do yourself a favour and buy a new system.