From my experience with programming I know that just throwing more memory against even a well paralized program will not speed it up after a certain limit due to various practical limitations (tasks can only be split in so many parts, waiting==processing or countless of other scenarios). Is there a limit at which which Adobe Premiere Pro doesn't speed up significantly anymore? Does it make sense to build a PC with 128GB or 512GB of memory? (My colleague is considering building a PC with 128GB and I am just a bit skeptical of this being... worth it, though I might be absolutely wrong as this really is not my area of expertise (and just for the record, I know that it's a 64-bit application and that it can address all available memory up to windows' limit, what I am asking is about the practical world)).
That entirely depends on what kind of footage he work with and how long his average cuts are. If we are talking hour long movies a bigger amount of RAM can be worth it. Though 128GB would be completely useless if he only uses Premiere it just doesn't use that much RAM. We built a high-end workstation once for huge 3D renderings that has 64GB RAM, the only times we utilize that fully is when working with gigantic 3D scenes in Cinema 4D or 3Ds Max. After Effects barley goes beyond 20GB unless you need an hour long RAM previews for some reason and Premiere is usually around 5-7GB even when cutting 60min plus movies. So if hes only using Premiere, 128GB would be a complete waste of money. I would not recommend anything more than 32GB on a video editing only workstation.
When using Premiere a high-end GPU is a lot more worth performance wise, Premiere can render in realtime with a powerfull GPU making having a ton of system RAM a bit useless.
I can recommend using gaming cards instead of workstation cards, the only difference is support and drivers, the gaming cards are much more powerful and a lot cheaper. I have never experienced any stability differences in Adobe software with a Workstation card compared to a gaming card. A good mixture of gaming GPU and workstation GPU is the GeForce GTX Titan, its meant for CUDA development and other CUDA usage.
Alternatively in the latest Premiere version there is also full OpenCL support, so a high-end AMD card would also be a good choice. F.e. The R9 290X or if he wants a workstation GPU for some reason the FirePro W9100 is currently the high-end model, though a waste of money in my opinion. You do not need 16GB Vram with Premiere. I would probably go for the W9000 or W8000.
Edit: As AJ Henderson mentioned, footage gets almost never fully cached in RAM so a fast SSD or maybe an SSD array will be much more beneficial than more RAM aswell. Going for fast disk I/O and a good GPU is a good combination nowadays. Having a high-end multi core CPU and lots of RAM is getting less and less important in the graphics world.
As non-linear video editing is random access by definition (that's where non-linear comes from), you will benefit from more RAM at least until you can fit all of your source footage in memory. The more data you can leave cached in memory, the faster access you can have to the footage whenever you need it. (I'm not sure exactly how Premiere handles this, but that is the only thing that would possibly benefit from insane amounts of RAM. If it isn't able to cache it all, then you would only benefit for however much it would bother to cache.) On the After Effects side of things, more RAM does mean more space for RAM previews.
Processing speed is really a very secondary concern when it comes to memory as things are a predictable and linear process at that point since the computer can anticipate what is coming and data access is not going to be the limiting factor at that point, but rather processor speed. DMA can retrieve data from the hard drive in to RAM as processing occurs and the processing of individual frames and groups of pictures isn't going to require that much memory space.
Some high memory consumption processing might benefit, but the primary use of RAM is going to be for holding assets in RAM for quick access while working with the footage.