The P4 1.4ghz will out perform both the P3 and the celeron easily, especially in multimedia apps such as adobe. The P3 1ghz is superior in every way to the celeron. Intel make the celeron for the budget range and sacrifice cache and other things while keeping relatively modest clock speeds, in reality the 1ghz celeron will probably perform the same as the 866mhz P3.
Going to the beach with a hammer and blowtorch and making a chip will probally result in a faster pc than a celeron anything. You are *way* better going with a low end xp chip.
A good way to test photoshop is generate a 250m file and then load it and run some filters and large actions on things and time how long it takes to do the various operations. For games, most have a builtin fps counter you can use to watch exactly how the game is performing.