9800 GT dedicated physx

It could possibly increase performance in a few games. However, to answer the specific game list you game, the answers are: no, no and no.

Only games that are specifically designed to increase performance with GPU accelerated PhysX will improve. There are only about 20 such games and many of those don't use a lot of power and can easily be handled on a single 470.

Note: while 3dmark Vantage does test physX, it will only test the graphic cards physX abilities and it will not be used to render the results during the test, so it would perform best on the 470.

I see, thanks alot for the fast reply

A single GTX470 is more than enough to play most of games with decent (high/maxed out) setting. And like bystander said above,only a little game that get benefit from PhysX card. A single GTX470 is already hot enough, no need to warm it again with another card...  

Since you already own both cards you might as well test it out your self and disregard what every one else here has to say as to their opinion and limited facts about physx. I have done this before and the 9800gt is one of the few sweet spots when it comes to dedicated cards but in most games there will be no boost but actually maybe even a small loss. In the end test it out your self and if you are not happy then stay with just that GTX470.

What I mentioned was not opinion, it is fact that a dedicated physX card does not help performance unless you are playing a game that supported GPU accelerated PhysX. The 3 listed games/programes he mentioned do not gain performance. It's not an opinion, but fact.

If he has one of those games that does support GPU accelerated PhysX, then I'd agree. However, he has not mentioned one.

You will need to test the PhysX performance in those games that use it. The ones you mentioned are not PhysX games. Recent PhysX games include Metro 2033, Mafia 2, Batman: Arkham Asylum, Unreal Tournament 3, Mirror's Edge, Dark Void, Cryostasis and a few others.