CPU:
No AMD or Intel preference, since my trusty 386 40Mgz I went back and forth between the two: I went from 386, 366 celleron, Athlon Thunderbird 1300 MHz, athlon xp 3000+, Athlon 64 x2 3800+, C2D E6450, C2Q 6600, I thin i'll go Phenom 2 next.
Mobo:
Asus, before that I was with DFI Lanparty. I love Asus, they have never let me down before, one of my Asus boards survived after I splilled a whole glass of brandy on it while it was running(twice in 3 month's, don't ask). If Asus let me down I'll probably go with Gigabyte or MSI next, heard good things from both.
Ram:
I'm not fussy about ram, usually kingston, but thats only because the place I work at alwys have Kingston ram, and I get ram wholesale price
Gfx:
I've used every known brand, but for ATI I prefer Saphire
Platform:
I like Windows and Ubuntu about the same, never used Mac so I can't comment on Mac.