I think Nintendo is just going to cause a lot of confusion having another Gameboy along with the SP and DS. Too many systems, too many names.... it's like the tail end of Sega with all their hardware (CD, 32X, Nomad, CDX, etc).
You forgot about the Saturn.
As far as Nintendo being comparable to Sega their are a few differences. Globally last year the SP was the best selling gaming machine- that includes outselling the PS2. None of the Sega systems on their downward spiral could ever make that claim. Next, the SP will be phased out when the next GB hits. We aren't going to be looking at three systems fighting for the same gaming dollar. Then, the next GB is going to target the same demographic that is the main focus of the current GB- younger gamers or those who want the simplistic/retro titles to tie them over on trips etc. The DS is targetted at a considerably older audience then either the SP or the PSP- they are going after the PDA replacement crowd. In that market, they are very cheap and offer considerable processing power compared to anything close to its pricepoint. They already have vid playback in the works for it also, I'm just not fond of the direction they are taking with it(they seem to be focusing on half hour to hour long program style media, fitting for the market they are going after).
In all honesty, the handheld gaming market is Nintendo's to lose. Pokemon is still the 800lb gorilla of the handheld market and Sony doesn't appear to have anything that can compete(the Pokemon franchise moved close to ten million units last year alone on a global basis, dwarfing GT and easily besting GTA).
Sony also has staggeringly huge problems with durability- and the fact that they already have a replacement program setup for defective PSPs in Japan(widespread sticking buttons) isn't helping their image there. Younger gamers are what pushed the original GameBoy to the position of best selling gaming platform to date- that demographic and fragile electronics don't go well together. Nintendo's handhelds take an incredible amount of abuse, as a father of four(soon to be five) I've seen the brutal abuse they can take and I don't hesitate to pick a Nintendo handheld up for my kids(my oldest got a launch DS and it already has numerous battle scars without a hiccup)- I won't even think about picking up a PSP for them. It's Sony, they are too fragile.
In my varrious daily reading I've been seeing a lot of ads for the PSP, and the most popular one I've seen states- "Because even your mom has an iPod"- I've yet to see any with a mention of the DS or GB. The PSP is a media device using the Playstation franchise to give it leverage. I am certain that SCE will do everything they can to push the gaming to be as succesful as possible, but they are fighting the 800lb gorilla of the handheld market with a nigh lock on everything they need to continue dominating the market. This isn't like the situation when the PS1 hit, Nintendo had already lost ~40% of the market by that point.
If the PSP is to become a success, which I think it stands a very good chance of doing, it will do so in large part by expanding the market and siphoning sales from other markets(Apple's in particular).
In no way do I mean to slam the PSP here, I'm itching to get it and am pestering my better half about it constantly(she's getting sick of hearing about it by now, trust me

). I'm already planning on dropping annother few hundred bucks upgrading my PSP(headphones, better battery pack, couple GBs of mem), I think the system will rock.