Screen Info:
With an ordinary NTSC television, the screen resolution is 480 pixels high (but without progressive scan, apparently only 1/2 of that is actually intended to be used), and the width... is a bit too complicated for me to try and figure out right now. However, the screen itself is laid out in a 4:3 ratio. That means that (however big the screen is) for every 4 "units" wide, it should be 3 units high.
"Widescreen" TV's use a 16:9 ratio, which basically means that they're almost twice as wide as they are tall. AFAIK, they have the same height resolution as a regular TV (480 pixels), but they have more width.
With the GameBoy Advance, Nintendo (not feeling bound by TV standards) put forward a "semi-widescreen" style that has a 3:2 ratio. The screen measures 61.2mm by 40.8mm. Measured diagonally, this is also called a "3 inch" screen. It's basically wider than a 4:3 TV, but not as wide as 16:9 widescreen. The screen resolution is 240 by 160 pixels. That height is 1/3 of the resolution of a TV, which (after compensating for the lack of progressive scan) is why the GameBoy Player games look perfectly fine on a TV when they're only using 2/3 of it's height.
With the Nintendo DS, Nintendo is sticking with the 3-inch 3:2 screens, but has bumped up the resolution to 256x192, and put in two of them, with one of them even being a touch screen. If one of these screens had been in the GBA, the GameBoy Player would have just about maxed out the width of the TV screen, without any need for zooming in.
For the PSP, Sony will be using the standard widescreen 16:9 aspect ratio. I've seen the screen called a "4.5-inch" (measured diagonally), but with a different ratio, it tough to say exactly how much bigger it is than the GBA's screen, physically speaking. If you're bored, algebra could tell you. Or you could just Google until you find the info online. The resolution is 480x272, which would mean that if you turned it 90 degrees, it would have more resolution (in either direction) than both DS screens (if they were stacked and combined).
Edit: While I'm at it, I should probably mention that the GBA SP was "front lit", meaning that (although the light itself was inside the unit) the lighting was in front of the liquid crystal display, and bounced off it, towards you, to make things visible. With both the DS and the PSP, we're moving into "back lit" screens, where the screen's pixels are like the little light-bulb pixels of your TV, and they generate their own colored light.