You're right, much of what you can do in Photoshop you can also do by hand.
I personally find that I work better if I do the initial sketch/line-art on paper.
The existence of layers, adjusting-tools for hue and saturation, the ability to UNDO things, and the free-transform (CTRL-T) + Warp tools are powerful things that make editing sooOOooo much easier than mere pen and paper could allow.
Consider Linux rather than Mac-- it's the same hardware as Windows, but the OS is free.
I use a dual-boot Win2k + Xandros Linux at home. The Xandros Linux runs most of my Windows programs (under WINE/ Crossover, a bit slower than on Windows) and has a mock-Windows interface which is so close to Windows that except for finicky hardware installs, one could actually believe one's in Win2k/WinXP.
Amen to not sticking with Windows, though-- Vista, the successor to XP, is more like an insult to consumers, shackles, and a "down"-grade to performance and users' rights. I wouldn't touch it with a ten-foot mouse.