Why? It's not like the 768 MB version is incredibly weak. It's just a better deal to buy the 1 GB as the slight extra cost buys you a relatively large additional performance. There aren't really any games out there that you can't play with the 768 MB model that you could with the 1 GB model.
Besides, I'm not sure if you'll have to pay anything to return it. Even if you did, shipping and restocking wouldn't cost that much. I don't see spending $120 on another GPU as an alternative to paying the small fees. I'd probably just keep the 768 MB. I doubt you'll see a difference.