My opinions in a nutshell:
Stock consoles:
DS > GP2X = PSP
DS obviously has the retail game advantage.
GP2X and PSP tie. GP2X may do more out of the box, but the PSP makes up for it with retail titles and Wi-Fi.
Homebrew modified consoles:
PSP > DS > GP2X
The PSP takes the cake because it's the most powerful of the three (it will emulate almost anything N64/PSX era on down at least near-full speed) and still has WiFi. (I know I'm bringing that up a lot, but it's useful.)
DS is weaker and its screens are smaller, but it still has its retail library.
GP2X fails because it has no 3D acceleration, no wireless connectivity, and costs more than its competitors.
SD = CompactFlash
SD is small, but CF is relatively cheap and pin-compatible with IDE.
Dell Axim X30 Mid = iPod
Sure the PDA does more, but the iPod can dedicate all of its storage space to music, while the 2 gig SD card in my PDA has to hold programs, notes, files, downloads, and browser cache.
<offtopic> I'm sorry but I don't buy the "Wii60" tale. Why do you need a "second system"? Are we that spoiled now, that we
need two consoles?
Calling it the "second system" is kind of grasping at straws, don't you think? </offtopic>