Upgrade from Pentium II 350MHz

it depends on the motherboard model, I need the name of the Manufacturer & Model of the motherboard.

Anyway, most Motherboard are good up to Pentuin II 450, but there are some Motherboards (newer ones) that support early Pentium III as well.

There were some boards with appropriate voltage regulators that could natively go to 1GHz, but again, you need 100 MHz bus CPUs.  But you really need to specify the board or at least the chipset to have a good idea of how fast you can get.

Further Information:

This Board: Intel Corporation MP440BX AA698435-206
Serial No.: ISMP82012221
Bus Clock: 100 MHz
BIOS: Intel Corp. 4M4PB0X1.15A.0013.P07
05/05/98

This was in a Gateway G6-350 system bought in August 1998 direct from the Gateway factory outlet here in Dublin, Ireland (since closed and gone!)

I built a new system last month and now decided to rehabilitate the bits left over.

It now has 256 MB RAM ( slots J6J 1-3; 64/128/64) - was 64 MB
Also 2 physical hard drives (C: 5GB this is the original drive; then  D: 10 GB; E: 30 GB )
Was on Win 98 now runs Win XP Home.

This was 'state-of-the-art' at the time. I'm pleasantly surprised it now takes XP, and it surfs just fine with a USB broadband router with IE6.

From what I've read Intel introduced a slot 1 processor when Pentium II was launched and I don't remember was there a change again with Pentium III - did they go back to a socket again?

Hope this additional information helps figure out how far I can push this old motherboard in terms of CPU and indeed RAM.

Definitely can handle 128 MB DIMMs and up to 550MHz P3 CPU.  The BX Chipset supports up to 1 GHz cpus, but the voltage regulator on the mainboard MUST support .18 micro based CPUs.  odds are, if that system was "state of the art" at the time of purchase, it doesn't.  Initial boards supported only the Katmai core (.25 micron dye).  As I mentioned, there were some add in cards that modified the voltage on the card and you could use socket 370 CPUs in them, but I'd suggest they are likely too costly.  Later boards included newer voltage regulators that could handle the .18 dye.

check this link

max'es out at 450 MHz

ftp://download.intel.com/design/motherbd/oem/69460501.pdf

but check if a bios upgrade can take it higher

More than likely you won't need the BIOS upgrade, the CPU will just work - the initial advertising specs concern then-in-production CPUs.  Provided the CPU is the same core as the previously supported CPUs, it will more than likely work - I will admit, there's a CHANCE it won't handle the faster (550MHz) CPU, but odds are strong based on my own personal experience that the 550 will work regardless of the manual's statement of 450 being the highest.  Note the earlier statement (around page 2 I believe) concerning processors does not include a limited speed.

I recommend a bios update.  I have an old bx board that orignally came with a p2 400mhz chip which i replaced with a p3 800mhz chip.   I did use one of those flip chip adapters though.  It is a slot card that has the socket 370 on it for the later model chips.  I did the bios update before I took out the old chip.  Put the new chip on the flip chip adapter, plugged it onto the main board, powered it up and all is well.  Even the bios recognizes it correctly as a 800mhz chip.  I think the board was a gigabyte board.

I also have a dually p2 motherboard in my server.  It orginally had a pair of p2 350s.  its a supermicro board which I updated the bios on before I put the new chips in.  i then put 2 slot p3 550's into it.  It worked flawlessly.

I don't that the bx chipset will work with 133mhz chips like the later p3's.  

I just suggest trying it without the BIOS update.  I've seen people screw boards with attempted BIOS updates that for whatever reason, they failed.  If the 550 doesn't work, then do the BIOS update - if there is one.

the board will only run at 100MHz.  _IF_ the board has the right voltage regulator, you can also use coppermine chips.  That said, if you use a 133 MHz bus chip, it will only run at 100 Mhz, so go out and buy a 1 GHz P3 coppermine and it will run at about 750 MHz.  AND you might have problems getting it to run in the board anyway.  (Remember, Intel locked their CPUs at Intel's desired clock multiplier.  So the Bus make a difference because you can't up the clock multiplier. (at least any way I've seen)

If all you can add is a pII 550, then you're going to see very little perfomance benefit in real terms.  Unless the processor is very cheap, I wouldn't bother.  What apps do you use?

Thanks to all who answered.

In the end I decided to leave well enough alone. Don't have enough experience to mess with BIOS upgrades on an old board. From further reading there appeared to be issues upgrading within Pentium II with S..E.C.C. and S.E.C.C.2 type cpu formats, differeing retention clips, heat sinks(?). I was about to buy a 450 MHz Pentium II but decided not to for the reasons mentioned and because it may not have made much of a difference.
The upgrade adapters appeared to be a useful alternative but at c. $100 plus shipping to Ireland probably not a good investment.

I did add RAM ( now 3 X 128 SD-RAM), 40 GB harddrive, a cheap 32MB PCI video card to replace a STB Nitro AGP card that I couldn't get to work after installing WinXP Home. It runs fine, plays DVD's, MS Office, surfs on broadband. Good to go for another few years!

Thanks all.

sounds like a sensible way to go.  It's really only gaming or specialist apps like vid editing that cane systems these days, and you'd end up spending a packet to get near the bleeding edge for these type of things.  
ta for the pts!