P4 800 FSB step down to 533/400 FSB?

While it might work, it will run much slower than it should. I recommend that you buy a CPU on the supported list (they are quite cheap on eBay). HT doesn't provide much benefits, if any.

This situation occurs quite often when a BIOS
pre-dates the vintage of the CPU:

If the BIOS does not recognize the installed CPU,
the machine will probably not POST e.g. no video.

For example, a larger L2 cache may not be supported
by an older BIOS version, but a newer BIOS version
may have been enhanced to support the larger L2 cache.


As the latter message recommended so correctly,
you should always start with the CPU Compatibility
List for that motherboard and its current BIOS version.

We just experienced this go-around at my office, when a
friend wanted to mix older P4s with spare motherboards
that he wanted to refurbish:

both aging Intel 550 and 631 CPUs failed in an ASUS P5GD1
-AND- in a brand new Intel DG43NB -- no video,
even though the LEDs on our Corsair XMS were
flickering normally when installed in the P5GD1,
indicating memory tests passed OK during POST.

BUT ...
... both of those CPUs worked AOK in an ASRock 4CoreDual-SATA2
motherboard that we picked up used for $35.00:

http://www.newegg.com/Product/Prod [...] 6813157115


AND ...
... both CPUs are also listed in the CPU Compatibility List
for a brand new ASUS P5Q-EM with Intel Q45 chipset:

http://www.newegg.com/Product/Prod [...] 6813131336


So, a faster CPU can still run at a slower FSB.

The BIGGER issue is whether the existing BIOS will
recognize the installed CPU.


MRFS

No. I understand what you are saying, but we are not
talking different technologies.

If his motherboard only supports 533 and 400 MHz FSBs,
but his CPU uses an 800 MHz FSB, those facts imply to me
that his CPU was manufactured much later than that
motherboard. I confirmed this by looking at photos
and it has 6 x PCI slots.

Accordingly, there's a very real chance that the BIOS
currently installed on that motherboard will NOT
recognize the newer CPU.

I've had this very same thing happen to me
several times, when trying to mix newer CPUs
with older motherboards.

One time, we had to purchase a cheap Celeron
just to be able to flash the latest BIOS,
which was needed for an E8400 to run in
an otherwise modern motherboard.

So, there are two issues in this thread:

(1) will the newer CPU even work and, if so,

(2) what performance penalty will it suffer
if it does work at all.

In my experience, those are two very different problems
that have occurred with both PGA-478 and LGA-775 sockets.


MRFS