actually, if you really look at overall sales,
Genesis > SNES. It sold more than the SNES why? because of EA sports games (madden). Overall, the Genesis had more marketshare than SNES because of the early entry. On top of that, the Genesis also had a FASTER processor than the SNES. However, the SNES had a larger color palette and a proven sound chip while sega stuck with their familiar "yamaha" brand. In the end however, deveopers to start understanding and figuring out how to finally push out better sound with comparable color to the SNES. But oh well, that was a few generations ago. Saturn ultimately did better than the PS in Japan thanks to it's Street Fighter Franchise that were better than the PS.
Problem lies here was that SEGA of America was dumb enough not to bring those games to the US. In fact, SEGA of America sucked so much that they released 2 editions of the saturn, one with the huge bulky controller, 2nd with the sleek 6 button designed for SF games that eventually made it's way to the US market (which by many considered those controllers to be the BEST street fighter controllers next to an arcade joystick). However, SEGA went the PS2-3 route with the Saturn, giving it 4 processors. That's right 4 processors. 3rd party developers were having the worse time coding for the system while SEGA pushed out games like VF2 (which looked amazingly close to the arcade MODEL-2 minus some objects) and eventually radiant silvergun (BEST Shooter IMO). Radiant Silvergun led to the new Ikaruga (which i still think Radiant Silvergun is still better) but they pushed massive poly's more so than the PS can dream of. However, because of it's difficulty of programming/developing for the console, 3rd party developers all left to the PS because they can produce similar graphics there with much more ease.
Dreamcast was born because of the PS, thinking that all it needed was ease of development. Of course, EA was upset at SEGA because of their own internal development team VC making NBA and NFL games. EA left, and went to Sony as a punishment despite the fact that Sony had 989 and other teams. (though 2nd party).
Anyways, this competition is no easy feat, and now it comes down to Sony and Microsoft. Rumor has it that even MS removed the HDD from their console.
Also, the power of the CPU's aren't even all that great. In fact, it's not even comparable to the AMD64's or the Intel P-D's. Remember the article that Anand took down discussing how the developers are complaining about the CPU's being "in-efficient" and "you get what you pay for" on the CPU side of things.