For a few years now Asus motherboards have accepeted processors from compatable families at ANY speed! YOu could have gotten a CUSL2 a while back, instead of the VIA board, or a CUBX, or a P2B, but right now the TUSL2 is the replacement for the CUSL2, it adds Tualatin support (yes, even at 1.6GHz). The only limmitation is the 512MB limmit of the i815EP chipset.
I looked at the pin chart for Socket 370, assuming the Tualatin has a 1.45v core, it can be modified to 1.65v detected core by jumping VID2 to VSS. That should be enough power to make it boot at 1600. The next one-jump mod takes it to 1.85v by jumping only Vid3 to VSS, but I think that might be too high (the Tualatin is supposed to have over-voltage protection).
What's the frequency, Kenneth?