There is also little point to buying a 8 series when they come out. No Vista = no DX 10, no DX 10 = little need for 8 series.
-cpubound
I don't know how immediate your need is, but let me suggest something: If you can wait till Vista hits, depending on your feelings about it. DX 10 will come with it, and a few DX 10/Vista only games. I don't see much point in buying a 8 series when they come out and have to wait 3 or 4 months for Vista to use them, that's just me though.
you can buy a 8 series and it will work perfectly fine in XP, but be aware you will never get DX 10 inside XP, it just isn't going to happen. In fact, you won't get anything new inside Xp DX wise. DX 9.0L is supposed to be DX 9.0c (basilcy) for Vista since DX 10 itself isn't backwards compatible.
back to the original question, video cards these days saturate the approximate level of PCI-E X4. THG did a comparison of frame rates last year or so, and they found that while the frame rate did change going from X16 -> X8, the drop was maybe 1FPS, and therefore not a bandwidth problem. Also the move from X8 -> X4 resulted in about the same loss, 1-2FPS. It wasn't till they got under X4 that there were issues. So waiting for PCI-E 2.0 is, honestly, ridiculous and entirely unnecessary. Hell, 2 X1950XTX's can't fill a X16 slot split into X8/X8.
No frets. Right now is the time to play the waiting game if possible, G80 is about to come out earlier than Vista.... big whoop, and R600 is on deck for release around the time of Vista, so I would wait to see how G80 and R600 break down against each other before I purchase one or the other.
As always, just my 2 cents