My computer won't boot up or something; no power led lit.

.Go to http://www.crucial.com/ and run their memory analyser. It will tell you exactly what you need

It has a Celeron M processor, with 1500MHz clock speed, 512MB of RAM and a HDD capacity of 40GB and a 15.4'' screen.

Well, Vista and Windows 7 require at least 1GB of system RAM to run, If I were you I'd think about upgrading with more memory. Otherwise you will experience a very slow PC, the RAM problem will then slow down your CPU and you'll end up with a big mess.

DDR1

Tough one, have you tried to get into safe mode? When you turn on the power switch on keep hitting F8 continuously until a command comes on the screen. It will give you options for safe mode where you can run checks and scans to find a problem. Either that or the hard drive died.
Hope it helps.

The system can use an AGP 4x video card and SDRAM PC133 memory. It support a max of 1GB of RAM.

*********** board will support a max of 1GB of RAM. Your processor is an Intel at 1.8 GHz. The preceding is only true if your PC is the Emachines T1840 as you chose in the category listing of your post. Again if that is true, you can achieve the max RAM by installing a 512MB stick in each of the two memory slots.
Good luck.

PS: Gdlkn84as528 is correct in assuming that a less than 1.3GHz Intel processor would be a P3 or equivalent processor. So which is it, it can't be both ways?

Phew, k3! A ton of questions and even more confusion going on here!

Firstly (love that non-word), your Motherboard accepts DDR Ram. Unfortunately, DDR2 will not be compatible. Won't work. Sorry. It does accept up to 2gb of DDR Ram, though.

Second, for Graphic Card slots, it has 3 x PCI slots (NOT PCI-Express), which are great for G-Cards. Maximum amount of memory won;t be affected by your MB, but by your processor and ram. As long as it has a PCI slot for a PCI card, you'll be good. Just be sure to turn off integrated graphics in BIOS settings.

Thirdly, good luck. Opening up the side of your PC can be daunting at first. Ensure you either have the patience to learn before you touch or have the money to continue learning as you go. >.o

all your using is outdated only you can use is your 40gb hdd for a new mainboard. your can go with intel core2duo cpu with ddr2 800fsb 2gb ram.

I have experienced something similar with my new Core2Duo processor and a fairly new Socket 775 processor. Troubleshooting it led me to believe the CPU simply wasn't compatible with the motherboard. Although it's a shot in the dark, I suggest that you try out some of the following: - Did you try pulling out the graphics card to see if you got it to POST then? The graphics card or the monitor may need replacing. - Did you remove the RAM? - Are you sure all cables are attached properly? - Try to take out the CPU, talk to a friend with a socket 478 motherboard and replace his CPU with your CPU. Does it work now? If yes, motherboard needs to be replaced.