If you are dead set on maxing out on the CPU, Few things I would look at. The user guide says it comes setup with Mobil P4, l1 8k, L2 512, 400 MHZ FSB Searching the online Dell documentation I can see no listing where they limit the CPU. Usually they say somewhere in the users or service guide. I did find this brochure: http://www.dell.com/downloads/us/products/latit/c840_spec.pdf First page on left says it runs at 1.2, 1.8 and 2.0 First I would look at the default processor as shipped. Go to: http://en.wikipedia.org/wiki/List_of_Intel_Pentium_4_microprocessors Scroll down to the bottom and you will see the Mobile P4 Processors. Look at your Thermal Design Power (TDP) setting for your as shipped processor. Actually I do not see a 1.2 listing, as you mentioned in your initial post. Later in the thread you mention a 1.6?? If it is a 1.6, I see a Northwood listed as 1.6 GHz, 512K L2, 400 FSB, 30W TDP, Socket 478 Notice the TDP setting of 30W. I would take that into consideration for which chip you ultimately purchase. All of the listings show nothing greater that 35W. So from the TDP standpoint you could use any of those, from 1.4 to 2.6. Stick within the listings. Personally, I would do what the others have recommended for the hard drive. Plus maxing out with 1024MB DDR. Then update to latest BIOS (A13) and then stick in the Pentium 4-M 2.6 SL6WZ. Update the Bios before attempting to swap processors. The Bios update (A12)on 5/22/2003, says in the enhancements it "Added support for newer revisions of the Pentium 4 processor" http://support.dell.com/support/downloads/download.aspx?c=us&l=en&s=gen&releaseid=R61061&SystemID=LAT_PNT_P4_C840&servicetag=&os=WW1&osl=en&deviceid=2286&devlib=0&typecnt=0&vercnt=9&catid=&impid=&formatcnt=0&libid=1&typeid=-1&dateid=-1&formatid=-1&source=-1&fileid=76752 I would install A13, 2/6/2004: http://support.dell.com/support/downloads/format.aspx?c=us&l=en&s=gen&SystemID=LAT_PNT_P4_C840&servicetag=&os=WW1&osl=en&deviceid=2286&libid=1&dateid=-1&typeid=-1&formatid=-1&source=-1&catid=&impid=&typecnt=0&vercnt=9&releaseid=R72793 Main Drivers and Downloads Link: http://support.dell.com/support/downloads/driverslist.aspx?c=us&l=en&s=gen&SystemID=LAT_PNT_P4_C840&os=WW1&osl=en&servicetag=&catid=&impid=&dateid=-1&typeid=-1&formatid=-1&source=-1 Memory can be maxed out at 1024MB @ 133MHz, 2 ea 512 sticks. Using DDR would equate to a FSB of 266 If you order through EBay, make double sure the "sSpec Number" is what you are ordering. The listings might say only a P-4 2.6 GHz. Is it a sSpec SL6WZ with a FSB of 400 or a sSpec SL724 with FSB of 533. Make sure it is the Mobile.