I was given 2 laptops, one with display or GPU issues with a 1.83Ghz T2400 in which a replacement display is $180 for this old laptop and the other runs perfectly fine, but it has a weak Celeron M 1.6Ghz Single-core.

Both the T2400 Core Duo 1.83Ghz and the 420 Celeron M 1.6Ghz are PPGA478 and are close in TDP wattage with the Core Duo 31watts to the Celeron M at 27 watts so I shouldnt have a heat issue, the Celeron M pumps out lots of heat and the laptop stays below 50C

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The uncertainty in the upgrade is with 2 areas...

#1 - Will the BIOS support the better dual-core CPU

#2 - There is a FSB difference between the two CPU's with the T2400 Core Duo 1.83Ghz 667Mhz and the 420 Celeron M 1.6Ghz 533Mhz

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In regards to #1, I assume that the worst that can happen is that the laptop fails to boot and then I put the original CPU back into it.

In regards to #2, has anyone ever run a faster FSB CPU at a slower FSB speed successfully. As for even if the Core Duo runs at an underclocked FSB, I am assuming that if the CPU works that the performance gain would still be worth the upgrade with 2 cores, and more cache.

Thanks for input on this. If it were a desktop system it would be an easier upgrade where a motherboard and BIOS version generally support a range of CPU's such as 533/667 or 667/800 etc, but when it comes to laptops that were designed to never have a CPU upgrade pretty much, I am in question as to if this would even work before opening the laptop and performing the CPU swap. I am almost thinking that the most probable upgrade may be to stick with the M series and get a Pentium M to replace the Celeron M with, but with a Core Duo already removed carefully from the troubled laptop and placed on ESD foam, it would be neat if it works. While I have upgraded many Desktop COMPUTER CPUs over the YEARS, Laptops always kept their original CPUs until they were removed from service.

Here are the specs on these CPUs:

http://ark.intel.com/products/27149/Intel-Celeron-M-Processor-420-1M-Cache-1_60-GHz-533-MHz-FSB

http://ark.intel.com/products/27235/Intel-Core-Duo-Processor-T2400-2M-Cache-1_83-GHz-667-MHz-FSB

As far as the Dell Latitude D520 laptop to be upgraded, it games ok for non complex games with its Intel GPU with 128MB RAM and 2GB system RAM, but the CPU's single core is pegged at 90-100% the whole time after clean build of Vista that it came with.I think I found my answer finally after about 45 min hunting for info... with BIOS Version A4, according to this link, the Laptop a Dell Latitude D520 supports up to 2Ghz and the T2400 that I have is mentioned as a working match.

http://en.community.dell.com/what-do-i-buy/f/3510/p/18379271/18543698.aspx#18543698Glad we could HELP.  Well that is my luck to hunt google and not find anything on it supporting yes/no on the upgrade. Post here for help, and then try a google search with different key words when thinking of a different way to word my question and get a hit finally  Better check this out first, sockets are not the same.
479-ball micro FC-BGA
http://www.cpu-world.com/CPUs/Core_Duo/Intel-Core%20Duo%20T2400%20LE80539GF0342M.html
478-pin micro FC-PGA
http://www.cpu-world.com/CPUs/Celeron_M/Intel-Celeron%20M%20420%20LF80538NE0251M%20%28BX80538420%29.html

See bottom of T2400 page:
Q: Can I upgrade my Intel Core Duo T2400 processor?
A: No. BGA processors, like this one, are soldered on a board, for that reason upgrading or replacing them is extremely difficult.

You'll have to actually look at your D520 to see what you have.  You could also enter the Dell Service Tag on the Dell website.  Enter Service Tag & click on System Configuration, which is the config that was shipped. Quote

You'll have to actually look at your D520 to see what you have.  You could also enter the Dell Service Tag on the Dell website.  Enter Service Tag & click on System Configuration, which is the config that was shipped.

...I will check out the Dell Service Tag though as you suggested to see what info they may have on this...