First, P4 1.5GHz only come with 423pin right, they don't make 478pin P4 1.5, do they. Second, 423pin CPU will not fit the 478pin socket, right. Third, if P4 1.5 only come with 423 pin socket. Then which board is better for it: Intel P4 (423Pin) D850GBC or Abit TH7.
Socket 423 is being phased out. Socket 478 is the newer version, but is no faster than socket 423 until the northwood chip arrives in a month or two. Socket 423 stops at 2 or 2.2 megahurtz, with no upgradability beyond. TH7 is the board for 423, TH7-II is for socket 478. Both are for rdram. If you overclock, I would go with the abit board. There are good system closeout deals on socket 423. And no, socket 423 and 478 are not interchangable. Socket 478 speeds range from 1.5-1.9, possibly higher.
Intel makes 423 pin processors at frequencies 1.3 GHz to 2.0 Ghz, and 478 pin processors from 1.5 GHz to 2.0 GHz, as of now. As far as socket 423 is concerned, 2.0 GHz is end of the road.
You should go for a 478 pin processor and the Abit TH7-II is one of the best one. I guess upcoming Asus P4T-E is the socket 478 version of their P4T board. It is the best one among the i850 motherboards. But with TH7-II you definitely have an option for onboard IDE-RAID, which I suppose will be present on the Asus board too.
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The 1.5GHz is indeed available in both 423 and 478 pin varieties. Since 423 is dead, go with 478, the 1.5GHz S478 goes for only $130 on Pricewatch. The Abit TH7-II is the choice right now for S478 motherboards.