GPU PhysX is pretty worthless, only really noticeable in two games.
Even then, your old 9400GT is FAR to weak to let you enable the extra effects.
Go with the 4650, you are not missing out on anything...
Just don't pay extra for a 1gb card if there is a 512mb version that is cheaper. The extra memory is entirely useless on that card.
Also if you can afford it and your PSU can handle it the HD4670 is significantly better.
I knew i did the right thing Bro.
after using it .. i am very impressed .. at least 60 to 70 % more powerful and i over clocked its core clock to 690 Mhz from 600 and memory clock to 550 from 500 giving atleast 10% more power.
also the CPU is e2200 which i OC to 2.9+ from 2.2 so all in all good med. mid range PC ... i am playing GTA 4 at high , COD MW2 at all hig 4xAA, Assasins Creed at high steeing NFS shift at all high ...and variuos other games...
NOW my PC :
ASUS mobo P5GC-MX/1333
WD 640 GB
2Gb Corsair 6400
HD4650 1 GB
Compaq Q2009 20` LCD