It's hard for anybody here to answer that question when the 9000 series is basically still non-existent at this point. There's a 9600GT out with some other stuff on the horizon, but if you're planning on building within the next few months, you're still going to want an 8800 series card.
If you want to play a game like Crysis on high settings with 1680x1050 resolution, you'll want to have an SLI configuration of two 8800 series cards. I personally prefer the 8800GT 512mb in SLI because of their price/performance ratio, and the fact that they're single-wide.
It's a bit ridiculous to come in here and ask what card to buy from a series of cards that isn't really even out yet, especially if you plan on building the computer in over 6 months from now, by which time new chipsets and processor architectures will become common.