Wait for the 8800. I've looked into it, there is nothing out there that's better.
And true, don't go out and buy an obsolete x1900, for that stick with the ATI 2600.
Even if you did find a cheap x1900 on eBay, most of them are from PCs. There is no guarantee that you will be able to flash it successfully for the Mac. (I've heard the success rate is 50/50, and you have to find someone with a PC that even has the graphics slot that you need. It's just not worth the trouble for an obsolete video card.
Ask yourself this: How long have you waiting for the new Mac Pro? I've been saving my pennies for a year. Now that the new machines are here and I have the money, I'm willing to wait another few weeks for top-of-the-line graphics.