Physx is only good for afew games and blowed up Vantage scores .. Ati runs the games just as well or better without it.. your powersupply need to be looked over to decide if you can run these cards or all you will get is reboots /BSOD/hard freeze..
You may even be able to run a 260GTX if the supply can take it..
Ok.. I don't know the different in U.S. and CAD but here in the states a 4870 is about $130-140 and a 4850 is about $110-130 so i would go for the 4870 as it would pwn both of those for the same price point.