I would be happy with a PSOne -- N64 (minus the CD-ROM disparity) or PS2 -- XBox jump.
To me that's a fairly sizable jump, games like Wave Race 64 and Rogue Squadron and Conker's Bad Fur Day on N64 were definitely noticably beyond what was on the PSOne. And DOOM 3, Halo 2, Riddick, were definitely beyond stuff on the PS2.
PS3/360 are third-gen 3D machines, so I think we're getting to the point today where yes you can always get better, but a a base, you can still make a very eye pleasing game even just that tech. I.E.: The Last Of Us, Witcher 2, etc. still look quite good on these machines.