Hard OCP recently did a DX9 vs DX10 comparison on Call of Juarez. They compared the 8800 series and the 2900XT... The 2900 really held it's own. Check it out:http://enthusiast.hardocp.com/article.html?art=MTM4NCwxLCxoZW50aHVzaWFzdA==
You will see that the 2900 performs above the 640 in DX9 mode. In DX10 mode the cards are neck-to-neck, but the 640 does get 1-2 fps better most of the time. When they crank up the AA, the 2900 doesn't back down. It does really well against the 640.
Another thing to note on this game: the 320 sh*ts a brick when you switch on DX10 mode. Seriously, it drops to 5 fps and they didn't even include the card in the AA tests (would have been pathetic). So if you have a 320, you're going to have to go with medium quality to run in DX10 mode. That sucks!
BTW, you guys have to check out the comparison screenshots between DX9 and DX10. HUGE difference. DX10 mode looks amazing, although it looks like they could have done a better job with the texturing in DX9 mode.