Different assemblers of the same graphics cards

I was wondering if there was any major differences between the different assemblers of the same model graphics cards? If so are the ones with faster core speeds (for example) better or worth the extra money or would you be able to overclock it on your own to reach the same speeds? Do the different heat sinks allow them to overclock the cards, is that how they get the higher core and memory speeds? I have notices that the "core speeds" and "memory speeds" are different from assembler to assembler.

Nvidia's new cards have no physical difference from various manufacturers except for maybe a different cooler.

When I removed the fan shroud on my 8800gtx I found an Asus specific sticker over top of a generic Nvidia sticker. The only Asus specific features of this card were the sticker, the bundle and packaging. When I bought my card the store had 3 different brands of gtx in stock and I bought the least expensive.
I say by from a reputable brand at a good price with a good warranty.

Rammedstein has covered it really well all i have to add is that yes the coolers do make a difference to how high you can overclock. thats not to say its limitless there is a physical limit to how high it will go but temp is a big killer of gpu`s so if you make it do more work it makes sence to put a bigger/better cooler to keep it at a good temp.
Mactronix

Thats true enough but from what i have heard the depending where you live bit comes down to if you live in north america you get a double warranty and if you dont you dont.
Anyone know why is it as simple as thats where the factory is.

Well not every manufacturer stays stock for example evga has replaced the plastic cover from the 8800gtx with a costom made aluminium one.