Most vanilla 6600's use the same core (NV43) as the GT's and can be easily overclocked (core) from the stock (~300MHz) to GT speeds (~500MHz). Memory, however, is another story, as Satunotaku has pointed out.
A quick example from a Guru3D article:
"The GeForce 6600 at default clock speeds is doing 300 MHz. It's DDR memory is 2x275 thus 550 MHz. Overclocked it was capable of running at 550 MHz core and 575 MHz memory frequency.
The high core overlock is something we observe with almost all 6600 cards, it's just amazing." http://www.guru3d.com/article/content/158/8/