it does not make a large difference as they all have the same core. I find EVGA, XFX, BFG to be good company's. As for the 256mbs of ram it really depends on the price difference but I would say yes fork out the extra for the 256mbs as long as it's within reason.
I, for myself, wouldn't buy such an pricey video card, but that's your choise. I also don't know the differences between the gt models. Probably some have faster memory than others and such.
About the 256 MB memory, yes, it is worth the few (dozen) extra pound. Games these days require humongous amount of video memory.
EDIT: How the heck... I was sure that the reply count for this thread was 0 when I posted, but what do I know...
EDITING AGAIN: But looky looky, you all posted right before me. What a dirty trick.
yeah I think you are seeing the 256mb 6600 non-gt, I'm pretty sure theres a big performance difference between 6600 and 6600gt, so it's probably best to go with the gt. On that note does anyone know why there isn't a 256mb 6600gt?