I got two (2) Simpletech 512M PC3200 RAMs in my PC installed in slots 1 and 2. I wish to increase my RAM capacity by adding a third 512M RAM  in slot 3.   However I have problems finding Simpletech RAMs locally (San Diego) without going to the internet. I understand to best use the same manufacturer in RAMs. Can I use a Corsair or Kingston or Mushkin instead?

Bubblehead

It's likely it will work, unless the ram architecture is very different I imagine. If it were mixed in a dual channel configuration it may not work AS dual channel though.

If your current rig is running in dual channel configuration, you could possibly create a performance DECREASE by adding the ram, creating a 3 module configuration, instead of the required 2 module configuration of dual channel performance mode.

It may even be prudent to purchase a decent quality 1024mb module, which you could pair up with another in the future, restoring dual channel mode and giving you 2gb in total.