In that case, NO it is not powerful enough for you.
The 4870 requires a 500w or higher PSU.
ATI Radeon™ HD 4870 System Requirements
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)
Look into a QUALITY 500w+ Corsair, PCP&C, Seasonic, Enermax or Antec Earthwatts PSU.