a 500w PSU is more then enough to run your ATI 4890 setup and just about any single GPU card out there. in fact a crossfire setup won't break 500w in total system power. I would suggest a unit from corsair, seasonic, antec, or enermax.
Processor(s): Intel i7 920 @ 3.8GHz (190MHz x 20)
Cooling: Noctua NH-U12P (Supplied by Noctua)
Motherboard(s): GIGABYTE EX58-UD5 (Supplied by GIGABYTE)
Memory: 3 X 2GB OCZ Technology PC-12800 DDR-3 8-8-8-24 (OCZ3G1600LV6GK)
Hard Disk(s): Western Digital 300GB Velicorapter (Supplied by Western Digital)
Operating System: Windows XP Professional SP2, Windows Vista SP1 64-Bit