I think for the best you need to look at either RDRAM or Granite Bay boards. They are the speed kings. I like the ASUS P4T533-C. Though if their Granite Bay one had been out when I was ordering I would probably of gotten that. For 512megs of RAM the price is very similiar if you look at MB plus RAM. once you get up to around a gig of RAM or more though the cheaper memory for the Granite Bay boards starts to make a pretty big difference. Performance wise RDRAM seems to have a little bit of an edge still but they (i850 vs. 7205 chipsets) go back and forth between benchmarks.
But for the best in P$ boards look at RDRAM or Granite Bay.