XFX doesn't implicitly state one way or the other for their 6xxx series cards...
http://www.xfxforce.com/web/support/sho...jspa?regionId=1&productGenerationId=40
"Additionally, third party products used in tandem with this Product are not covered by this warranty." - Doesn't say you can't use an aftermarket heatsink, but the heatsink is not covered by warranty.
"XFX reserves the right to claim shipping fees as well as a service charge* from the purchaser (owner #1 or #2) for any incomplete or modified Product that is returned and that requires repair or replacement, or when the Purchaser (owner #1 or #2) is not entitled to any coverage under the XFX Double Lifetime Warranty." - Your card will still be covered, but if you send it to them without the original heatsink, they'll charge you for it.
So, seems like a bit of a gray area. My view of this warranty is, if you were to replace the heatsink, then have to RMA it, you should replace the stock heatsink. Possible that they could still tag you with the "modified" charge, but doubtful since at no point do they say anything to the effect of "Modifying your card with a 3rd party heatsink will void your warrantee."
If, however, you damage your card while installing a 3rd party heatsink, you're SOL.
-z
EDIT: As far as overclocking, that will void your warranty.
"Other situations that VOID the warranty include:
...Was not operated in accordance with XFX specifications, instructions or technical support directions.