Yes, you can replace the motherboard. Here's a link for that type of mobo to an online store that I think most people on these forums will agree is the best for buying computer parts:
On the other hand, that motherboard, which uses a Socket A processor, is extremely outdated. It would be good to upgrade to at least socket 939 and preferably socket AM2 or 775 if you have the money.
To change from 16 to 32 bit color, right click the desktop and choose properties. Go to the settings tab and change the Color Quality to 32 Bit.
Is the computer doing anything at all? Does it make any noise, movements, anything? If not, then yes, it probably is the power supply. First though, check all the connections or the cables inside the computer case. Something might have gotten loose and is causing a problem. Is the computer a standard size (ATX Form Factor) case? If so then you can use any standard power supply with it.
Do you know which version of Linux it is? There are many out there (ex. RedHat, Ubuntu, Debian to name just a few)
You need different drivers for hardware with Linux computer than a Windows computer. Linux and Windows understand different programming languages, so you need different programs. Drivers are the software that makes the hardware work (they "drive" it). As for the printer drivers, here are the 2 websiotes you will need to download drivers from:
The monitor should work with no set-up, if you have problems, post a new thread about it.
Most files in Windows won't be able to be read in Linux.
Yahoo messenger has a Linux version here that supports Red Hat, Debian, and FreeBSD. I assume that it has sound and webcam support just like all the other versions.