Dimension 5150 not booting - LED's 123 on

Did you replace the cmos battery?

http://support.dell.com/support/edocs/systems/dim5150/en/sm/tshoot1.htm

dim5150
dim5150C

Remove all cards.

Remove all Ram.

Try 1 stick in 1 slot.

Then what happens?

Another failure has occurred.

• Ensure that the cables are properly connected to the system board from the hard drive, CD drive, and DVD drive (see "System Board Components").
• If there is an error message on your screen identifying a problem with a device (such as the floppy drive or hard drive), check the device to ensure that it is functioning properly.
• The operating system is attempting to boot from a device (such as the floppy drive or hard drive); check system setup to ensure that the boot sequence is correct for the devices installed on your computer.
• Check the computer message that appears on your monitor screen.
• If the problem persists, contact Dell (see your computer Owner's Manual for information about how to contact Dell).

Memory modules are detected, but a memory failure has occurred.

• If you have two or more memory modules installed, remove the modules, reinstall one module, and then restart the computer. If the computer starts normally, reinstall an additional module. Continue until you have identified a faulty module or reinstalled all modules without error.
• If available, install properly working memory of the same type into your computer.

Forgot to mention - changed CMOS battery too...

Just to spite me, the PC is currently booting fine, will investigate further and report back when it goes gaga again - though from the above, it seems most likely that it may be RAM related, as it is too early in the boot process to be detecting drives, or attempting to boot....