From what Ive read, the biggest performance increase is from overclocking the memory and no the GPU. I here its common to get a retail (275/275) up to 300/300 with the orb and ramsinks. Whether or not that is worth the effort with the upcoming Radeons clocked at 300/300 will be determined by the price of the new cards.
Personally, Im waiting on the new line of ATI cards (hopefully announced in the coming week during CeBit), to drive the price of the 8500s even lower for me to purchase (and then overclock).