Graphic card Performace

[jackstuff]

Hey Guys I want to buy a new graphic card i got messed between Ati & Nvidia Series, I have also googled them but can't fixed the appropriate once, so which will be better on performance level as well as price
My System Configuration Are As Follows:
Amd Phenom 6x 1055t Processor 2.80ghz
4gb ddr3 ram run on 1333mhz
600watt pure power supply
am3+ GIGABYTE motherboard
500 gb hdd
nvidia gt 240 series graphic card
17" led hd monitor

I have already use nvidia & wanted to buy Ati card with a additional power supply port on it, nvidia is fine for me but on mid level setting on games it seems to be hanged and get slow down, so which one i should buy under 10k
Note: Additional Power Supply Port Should Must