Good Dedicated PhysX card with a R9 270X (7870)?

I got a Prebuilt PC from CyberPowerPC (Dont worry, i saved money by NOT building by abusing there discounts) and its coming with a R9 270x which is a rebranded 7870. But i plan on playing Planetside 2 ALOT. and i plan on getting into Borderlands 2. So i could use PhysX in my build. I was looking at a Geforce GT 640 because it doesnt require a cable from the powersupply (Mother board supplies enough) and i am worried if it will bottleneck my 270X. So should i dish out 20 bucks and get a 650 (Re-organize cables -_-) and risk blowing up my power supply (Corsair CX-600) or will the Geforce GT 640 have enough juice for medium or high PhysX in planetside 2 and Borderlands 2? I will also most likely play the batman arkham games (I predict steam will sell a nice bundle with the release of Arkham origins). So yeah, i could use help. Or should i FORGET about PhysX Completely? I really think it looks cool and would it be worth messing with the cracked Nvidia drivers? Thanks in advance