Mac total shut down

 ???ok.. I have and eMac. I know that it's kinda old, but that's besides the point...  this computer never had any problems until i decided to get a video ipod.  To support the ipod, i had to upgrade from its original running system, to mac-osx (leopard)...  $100 and a new memory card later, my ipod works great.  the problem is, my computer doesn't....  I didn't notice any problems until a couple months later... NOW!, I cant even load a single page or program until the previous one is done loading. (If i do anything while a file or internet page is loading, the computer locks up and i have to reboot). It is frustrating to even look at this computer. I'm not stupid with computers, but i really don't understand what  this thing is doing!  Please help... I want my computer back!.. 

I think my old mac was an Emac too. the one with the 360 super swivel monitor??? Sorry mate I think this computer is teh deadzorz. (translation: The computer is dead most likely) My mac died, . So i got a PC.
Try taking it to your local computer store. but, if you have any smart computer friends, take it to them.

a new memory card later

By this, do you refer to new RAM in the system?

The fact that the computer won't "multitask" applications (locks up if one is initiated until another is done) makes me think it's a problem with the Random Access Memory.

Unless, of course, it's a problem with the OSX installation, itself...

???ok.. I have and eMac. I know that it's kinda old, but that's besides the point...  this computer never had any problems until i decided to get a video ipod.  To support the ipod, i had to upgrade from its original running system, to mac-osx (leopard)...  $100 and a new memory card later, my ipod works great.  the problem is, my computer doesn't....  I didn't notice any problems until a couple months later... NOW!, I cant even load a single page or program until the previous one is done loading. (If i do anything while a file or internet page is loading, the computer locks up and i have to reboot). It is frustrating to even look at this computer. I'm not stupid with computers, but i really don't understand what  this thing is doing!  Please help... I want my computer back!.. 

I would have highly recommended upgrading to OS 10.4 (Tiger) instead of 10.5 (Leopard) in your case.  Leopard requires some pretty new hardware, and quite frankly, your emac is very old.

However, what's done is done.  I would recommend getting some more RAM for your system.  What does yours have currently?