I've now stuffed the motherboard with the most CPU cores that it can handle with the X6 (though not the fastest CPU it could take - that's actually an X4 at the moment) and with all the RAM it can handle. The next upgrades are not urgent, or should not be. I have a choice of which way to go, but both are video.
Right now I am using a Samsung SyncMaster 930B monitor. It's a 19-inch diagonal LCD in the 4:3 (non-widescreen) format. If/when I replace it, it will be with something widescreen and at least 25-inch diagonal. Anything less will be physically shorter and I know that will bother me even with a higher resolution. I am considering going to 27-inch - why mess around? I'd want LED backlit LCD and some sort of digital connector. DVI or HDMI will do.
I am also using an ATI Radeon HD 3870 video card (256-bit interface to 512 MB GDDR4, 320 stream processor, 16 texture/render units - if I am reading the right specs correctly). I could upgrade that. But to what? First, I would not go with another ATI. ATI might win benchmarks but I won't be at the expensive high end anyway. I will NOT be buying any $700(!!) video card or such. nVidia has open source drivers that work and work well. ATI... doesn't. And ATI's proprietary Linux drivers... give one good reason to seek open source alternatives. Allegedly those are coming, but they aren't here yet. Also, supposedly nVidia does OpenGL better and that's something that has become important to me. Thus nVidia wins.
But even so what parts matter? Does the amount of memory matter? Or rather, where does it cease to matter very much? Is there significant gain from 512 MB to 1 GB? Is there similar gain from 1 to 1.5 GB? From 1.5 to 2 GB? Is the big thing the move from GDDR4 to GDDR5? Or does it not even matter much and the number of stream processors or texture renders more important? Or is it some complex formula and there is no simple answer beyond "Get as much as you can afford"? And there is the possibility that I really wouldn't get any significant gain from a change at all.
Assuming that a video card upgrade is a Good Idea, is it better to do that first or would I be better off with a larger display first?
 How long before the icon with the CRT monitor switches from being uncool obsolete to being quasi-cool retro?