"The PS3 is 128 bit, but it is more 128 bit than the others. The number of bits isn't really a very good measure anymore. To be honest, it hasn't been a good measure since PS1 days. That said...
Most single pieces of data fit in 32 or 64 bits. The benefit of 128 bits is that you can operate on 4 pieces of 32-bit data at the same time, which is called SIMD (Single Instruction, Multiple Data). This is only useful for data that needs the same operation on all 4 pieces, which is common in games for things such as 3D graphical transformations, physical simulation, collision detection, etc. 128-bits is the "sweet spot" of price and performance, so that is what everyone seems to have settled upon.
To get more power, people have instead now been moving to more processor cores. (PS3 has 8, Xbox 360 has 3, Wii has 1, PS2 had 1 + 2 special-purpose, Xbox had 1, etc).
For graphics, it is even trickier to explain. The biggest difference is that in the past, graphics chips were "fixed-function". Now, they are programmable. But people don't really talk about it in terms of bits; instead, they usually measure in terms of flops."