Brewology

Yep...they have been asked plenty of times. No you only need to install one version of custom firmware. When you downgrade to 1.5 and upgrade to a custom firmware, its essentially the same as upgrading to that identical Sony firmware but with homebrew capabilities. And the 2 most common ones used are 3.03OE-C and 3.40OE-A. For ps1 emulation, 3.40OE-A is better. 3.40OE-A also plays more game(s). But 3.03OE-C is easier to customize as in, customizing the xmb. And there should be a readme included in the file you download that contains the custom firmware that will have instructions on how to upgrade. Or just briefly browse through the tutorial section here.