How long ago did you buy the card?
They'll only give you a replacement to the value of the card now... i.e.: if the 8800 is only worth £50 now then that's all they'll put towards a replacement.
I got the same deal when my card fried, got offered either a like-for-like, or £70... was a tad pissed, seeing as it cost me £350, and the 70 barely paid for the warrenty I bought.
I've no idea how the two cards compare, or what the cost of each is, but don't expect something of the same value as you originally paid.