Recently I've increased my workstation RAM from 2GB to 4GB (all modules are CL4). Although I am running Vista Ultimate x86 and the available RAM increase is actually smaller than 2GB (Task Manager is reporting 3581MB of total RAM) due to x86 nature of OS the speed increase is quite nice. No, I haven't done any benchmarking or anything, I am speaking out of my subjective experience. Was it worth? Definitely. It would be nice to have all 4GB available, but hey, I really don't want to go x64 path (where I would have 4GB available and possibly more), not right now as half of my devices wouldn't work due to the lack of proper 64 bit drivers. Yes, the x64 era for workstations isn't yet here. Unless you really really really need more than 3,5GB RAM I don't see a reason to go with x64.
The bottom line is that if you are doing development (or any other serious job that consumes memory) don't hesitate and jump on 4GB wagon even if you are on x86 OS. Perhaps a word of caution here: some (older) motherboards have problems with 4GBs and you might get a lot less available RAM than me, so check it out before buying RAM. And if you wonder, my motherboard is Gigabyte GA-965P DS4 rev 1.0.