I have a Shuttle FB51 computer (845GE chipset). It currently has a
Northwood P4 socket 478 cpu running at 2.4Ghz. I want to upgrade the
CPU to a 3.2Ghz Prescott with HT. I want to know if I need to purchase
a 533Mhz FSB version of the chip or can I get away with an 800Mhz FSB
chip. That is will an 800Mhz FSB chip run at 533Mhz (i do not want to
overclock). I am running PC2100 RAM at 100Mhz.

first of all you'll need to update the BIOS on the motherboard in
order for the Motherboard to recongise the new Hyper Threading CPU and
then you can use either 800FSB or 533FSB CPU on your M/B.

Hang on..

You are running PC2100 Ram at 100Mhz and you want to run at 533MHz FSB?
To my knowledge, 533Mhz FSB requires you to have DDR2 Ram. 
Are you sure the 3.2 Prescott is also Socket 478? (Sorry havent done
research on that)
Are you sure your motherboard can handle the 3.2Ghz Processor? assuming it is s478
What is the Speed of the Motherboards FSB?
Can your motherboard accept DDR2 Ram.

800FSB Also requires to have a minimum of DDR3200/200Mhz Ram.

I think you will need to upgrade more then your processor.
Also. HyperThreading means it CAN RUN at 800Mhz FSB. If you disable
Hyperthreading through the bios, it will run at your ram/motherboards
default

The FSB is something that is determined by the BIOS. If the max for
the motherboard is 533 then the Prescott will be underclocked
automatically to run at 533Mhz.

I don't know how you are running PC2100 RAM at 100Mhz by the way. It
should be running at least 266Mhz. I can't remember exactly how fast
PC2100 is.

Hyperthreading really has little to do with FSB and more to do with
the internal workings of the CPU.

Nothing will be damaged by putting the new CPU in there but it is
possible that the CPU will not be recognised by the motherboard so
make sure you have the latest BIOS version.

A few thoughts...

A.  PC2100 RAM runs at 266mhz.  I suspect that this was a simple
mistake as its is IMPOSSIBLE to underclock RAM that much.

B.  The chipset on your motherboard is in compatible with a Prescott
core.  The prescott requires an Intel 865 chipset or better.

C.  If you try and put an 800 mhz processor in a 533mhz motherboard
that does not support the higher speeds the comnputer WILL NOT WORK. 
The only way to upgrade your chipset is to get a new motherboard.  If
you choose to go that route I would go with a newer socket, ie:
LGA775.

Hope that helps...

I put an 800mhz pentium 4 socket 478 with HyperThreading in an ASUS
P4T533-C Mobo which only runs at a MAX of 533mhz.  It works.  XP says
it is only running at the max of 1.7Ghz, but when testing the speed it
is acually running much faster.  HyperThreading is also working just
fine, no problems.  You CANNOT overclock ANY with this kind of setup
on THIS particular ASUS board.  Good idea to have full, 100% money
back return privileges just in case this does not work on YOUR MOBO.
The answer in my case was YES you can run an 800mhz HT socket 478
processor on a 400/533Mhz MOBO.
IMHO isepiq