Now THAT, is a proper RMA.
That's actually a good upgrade from your 8800GT. It's a whole new chipset too, one which is pretty reliable. I don't foresee you having problems later on.
And yes, as mousemonkey pointed out, the 8800GTX used the "G80" chipset, which was the version PRIOR to the "G92" in your old 8800GT. Ultimately, the G92 revision was a faster/better GPU.
You're good to go with that new GTX 260. That's a good card.
Edit was to fix typographical errors.