If you have one of these Laptop models:

eMachines:
M6805
M6807
M6809
M6810
M6811

Gateway:
7405GX
7410GX
7415GX
7422GX
7426GX
7505GX
7510GX
MX7508
MX7515

All these models come with PC2700 DDR333 MHz RAM Modules. There are two slots, one on the underside of the laptop that is very accessible through a RAM slot door. The other module is located under the keyboard that requires a lot more work and will void warranty.

All these models can be upgraded to 2GB of PC2700 DDR333 MHz 200-Pin RAM, OR 2GB PC3200 DDR400 MHz 200-Pin RAM, that's it, no DDR2 PC3200 400MHz, PC4200 533MHz, or PC4300 533MHz, these WILL NOT FIT OR WORK.

I have personally upgraded 3 machines, my 7422GX from 1GB PC2700 to 1GB PC3200 (2 512MBs), an M6805 with PC2700 512MB (2 256MB Modules) to 1GB PC2700 (2 512MBs), an M6810 with 512MB PC2700 (2 256MBs) to 2GB PC3200 (2 1GBs). I've used Centon PC3200 myself, all these systems came with Samsung PC2700 RAM so chances are your's does too.

If you do not want to take your laptop apart to get to the slot under the keyboard, if you have a 512MB laptop, that means you have 2 256MB PC2700 RAM modules, you can keep the inaccessible 256MB, and upgrade to either 1GB, or 512MB RAM to make 1.25 or 1.5GB RAM. Remember that you cannot interchange PC2700 and PC3200 RAM to attain 400MHz, that means if you stick with the inaccessible 256MB or 512MB PC2700, you cannot get a PC3200 stick to run at 400MHz, it will run at PC2700 333MHz speed. You have to stick with the same types of RAM for both slots, or the chances are it won't work.

If you have any more questions, post here. I've seen a bunch of threads asking about memory upgrades about these models and I figured one thread can do the job. If I made a mistake or want me to add anything, let me know. And for those who have successfully upgraded, please post your comments and what brand and size and type of RAM you chose.