If you want PhysX that badly then get a GT240 or higher, the CPU will get so bogged down it just isn't funny and it's not because the CPU can't handle the workload but more to the fact of the way PhysX has been coded.
If you have an nVidia card it will slow down if you ask it to render physX and graphics. The Fermi cards handle the switch between the workloads better, but unless you have something slower than a 9800GT it is better to have PhysX run on the GPU than on your CPU even if that CPU is an i7. It's just the way nVidia crippled physX on the CPU to get you to buy more of their video cards .
Anyway, if you just really really really want a boost in the few games that use physX then get a 9800GT (or if you want a faster GTS 250), to use for physX.