http://www.ldlc.com/fiche/PB00099394.html
Voilà une HD 5870 abordable, pour une upgrade pas cher 
Quant à ton alim, ça suffira :
ATI Radeon™ HD5870 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard.
500 Watt or greater power supply with two, 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four, 75 watt, 6-pin, PCI Express® connectors for ATI CrossFireX™ technology in dual mode.
The power supply recommendations are for an entire pc system.