Depends on your budget. Geforce 6600 (make sure it's not 64-bit memory interface) or 9800 Pro, 6800 (128 MB), 6800 GT, 6800 Ultra or X800 XT PE, depending on what you can afford. Best-best would be the $500+ 6800 Ultra or X800 XT PE, not going to get into the endlessly retarded debate about which one's better unless somebody actually has consitent benchmarks for EQ2.
(Which would be almost impossible.)