I have a number of Dell Optiplex GX270 systems without processors and
would like to know which processors I can put in them.  The original
processors were socket 478, Pentium 4, 2.4GHz to 3.0GHz with 533 FSB. 
I would like to put in some of the newer 800 FSB Intel processors with
HyperThreading support.  And put in some of them the cheapest
processors I can find (e.g. 1.5GHz Pentium 4).

Two questions:

First, in general what processors are compatible with a Pentium 4
533FSB, socket 478 processor?

Second, will the Dell GX270 motherboard and BIOS support the
processors you identify in the first question?

Please include in your answer web sites that I can read to get full
details on the answers you provide.

kent...

"First, in general what processors are compatible with a
 Pentium 4 533FSB, socket 478 processor?"

It's more a matter of what processors are compatible with
a socket 478 motherboard, which depends to some extent on
the chipset it uses.

This page on ABIT.com notes:

"Socket 478 is a type of CPU socket used for Intel's Pentium 4
 and Celeron series CPUs. has been used for all of the
 Northwood Pentium 4s and Celerons, the first Prescott
 Pentium 4s, and some Willamette Celerons and Pentium 4s.
 Socket 478 also supports newer Prescott Celeron Ds, and
 early Pentium 4 Extreme Edition processors with 2MB of L3
 cache. The socket was launched with the Northwood core to
 combat AMD's 462-pin Socket A and their Athlon XP processors.
 Socket 478, which accommodates high- and low-end processors,
 was also the replacement for Socket 423, a Willamette
 processor socket that did not have a long life."
http://www.abit-usa.com/products/mb/socket478.php

That page has a link to their Intel CPU Support page.
Upgrading to CPUs with 800 FSB seems doable. According
to the chart on that page. For their Socket 478 Motherboards,
you can go as high as the 3.4GHz P4 with an 800 FSB:
http://www.abit-usa.com/products/mb/intel-p4.php

According to this article from Xbit Labs, there are no 
plans to make faster CPUs available for this chipset:
http://www.xbitlabs.com/news/cpu/display/20040123103754.html


A search for the GX270 and 'chipset' indicates that it uses
the Intel 865G chipset.

A search for that chipset shows that this chipset supports
a wide range of processors, as in this board described on
Geeks.com:

Supported Processors:
- Intel Pentium 4 GHz 1.40 - 3.06 GHz Socket 478 processors
  w/400/533 MHz FSB
- Intel Celeron 1.70 - 2.80 GHz Socket 478 processors w/400
  MHz FSB
http://www.geeks.com/details.asp?invtid=D845PESV-WB&cat=MBB

[The 3.06 limit was likely placed there before the 3.4GHz
 processors were created]


A search for "Intel 865G chipset" "supported processors" on
Intel's site produces a board with a similar description:

"Support for an Intel® Pentium® 4 Extreme Edition processor
 in an mPGA478 socket with a 800 MHz system bus

 Support for an Intel® Pentium® 4 processor in an mPGA478
 socket with a 400/533/800 MHz system bus

 Support for an Intel® Celeron® D processor in an mPGA478
 socket with a 533 MHz system bus

 Support for an Intel® Celeron® processor in an mPGA478 socket
 with a 400 MHz system bus"
http://www.intel.com/design/motherbd/bf/


So it also looks like you can also downgrade to the cheaper
1.5GHz P4 CPUs with 400MHz FSB, such as this one at Tom's
Hardware, or even to the supported Celeron CPUs:
http://stores.tomshardware.com/search_techspecs_full.php/masterid=527841


As for the BIOS, it is tailored to the motherboard, so if
the motherboard will accept the chip, the BIOS will be able
to work with it.


Please do not rate this answer until you are satisfied that  
the answer cannot be improved upon by way of a dialog  
established through the "Request for Clarification" process. 
 
A user's guide on this topic is on skermit-ga's site, here: 
http://www.christopherwu.net/google_answers/answer_guide.html#how_clarify 
 
sublime1-ga


Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.

Searches done, via Google:

"Dell Optiplex GX270" processors
://www.google.com/search?q=%22Dell+Optiplex+GX270%22+processors

"Dell Optiplex GX270" chipset
://www.google.com/search?q=%22Dell+Optiplex+GX270%22+chipset

"Intel 865G chipset"
://www.google.com/search?q=%22Intel+865G+chipset%22

"Intel 865G chipset" "supported processors"
://www.google.com/search?q=%22Intel+865G+chipset%22+%22supported+processors%22

"Pentium 4 1.5GHz" 400 FSB "socket 478"
://www.google.com/search?q=%22Pentium+4+1.5GHz%22+400+FSB+%22socket+478%22

"socket 478 processors"
://www.google.com/search?q=%22socket+478+processors%22

"Intel Pentium 4 Socket 478 processors"
://www.google.com/search?q=%22Intel+Pentium+4+Socket+478+processors%22

"Intel Pentium 4 Socket 478 processors" 400 FSB
://www.google.com/search?q=%22Intel+Pentium+4+Socket+478+processors%22+400+FSB

"socket 478" "Intel 865G chipset"
://www.google.com/search?q=%22socket+478%22+%22Intel+865G+chipset%22

"socket 478" "Intel 865G chipset" "Optiplex GX270"
://www.google.com/search?q=%22socket+478%22+%22Intel+865G+chipset%22+%22Optiplex+GX270%22

Excellent answer with plenty of detail and references.  Thank you very much.