Very interestingly, the high ISO performance of the EOS 400D is really excellent!!!:OI cannot express to you how good it is.
If high ISO performance is what the OP wants, he's best off buying the 30D. He will also get a much better viewfinder, much better burst mode, and much better build quality.
But, I don't think ISO1600 is what anyone should base their decision on. There's so much more to a dSLR.