A sub 3GHz non hyper threaded 533MHz effective FSB P4? I'm not even sure I'd go that high. 5570 or GT220/9500GT might be a better fit.
I kinda agree that a P4 2.66GHz CPU will bottleneck a HD 5670 a little, but it should still perform better than a HD 5570. Since the price difference is only $12 (after rebate and shipping), I would go for the HD 5670.