Yes, the Antec Earthwatts 500w PSU will be fine for that system with that card. It even comes with the 2 6pin PCIe connectors
Minimum System Requirements for a Sapphire HD4870
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
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)”
Certified power supplies are recommended.
1GB of system memory
Installation software requires CD-ROM drive
DVD playback requires DVD drive
Blu-ray™ / HD DVD playback requires Blu-ray / HD DVD drive
For a complete ATI CrossFireX™ system, a second ATI Radeon™ HD 4870 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per board (included) are required
Since you have minimal drives and dual, not quad cores, you will be OK... Since it's a decent antec unit. Of course, the preferable thing would be to spend $60-80+ on a higher wattage PSU, but you will be fine, especially if you don't overclock.