The HD 2900 XT isn't a bad card at all, after a few releases of drivers it really gained some good momentum and you can see that it really isn't that far behind the new cards. The problem with it now though is that it's a rip off when you look at its pricing off the shelf. On the plus side, when you jump onto the second hand market you'll notice all of a sudden that the HD 2900 XTs are for sale at ridiculously low pricing.
The 8800GTS has really dropped in price and is worth next to nothing on the second hand market. Right now it's an excellent chance for people to jump on the Crossfire or SLI bandwagon on the cheap.
The HD 2900 XT 1GB from Diamond is a great card, the pre-overclocked speeds on it give it a nice boost over a stock clocked HD 2900 XT, even bringing performance up to and above the 3870 a few times.
We're looking forward to checking out what the Diamond 3870 brings with it, and until then there is no doubt a lot of people out there are going to be able to pick up a real bargain.