The tasks you listed are mostly for build machines, not development machines.
- do you think I should go for 3 monitors or just stick to 2?
For a build machine, none.
If you are doing development on it, 2.
- I was thinking for a big HDD for OS and apps and then a separate small but high speed SSD exculsively for compiler and source code (for quick linking times)
For fast build times, keep the build on SSD if you can. Otherwise, use the hybrid drive technology that comes as part of your computer. Both AMD and Intel have technology and drivers that turn your SSD+HDD into a hybrid drive. That is, part of your SSD (e.g. 64GB) is reserved for caching, and stuff that is frequently accessed on the HDD gets mirrored in that portion of the SSD. On a cache hit the performance is the same as the SSD, but you still get the massive (terabyte+) storage capacity of the HDD. Also for SSD, get the kind that plugs into the system bus as a card, since that is faster than the ones on the SATA controller.
Even though both companies include it, a discouraging number of people with SSD+HDD systems never properly enable the functionality, and consequently never get the major performance boost that is sitting on the chips ready for use. All of my home computers and most of my work computers have that setup. Some of our work computers are mis-configured, and they have much slower average disk times in the typical case as a result.
- CPU I think 8 cores maybe (for compile times)? But I would not want to sacriface single core performance since it helps me greatly when testing (the game launches and initializes faster, I'm using single core code in my games). What clock speed should I aim for?
I'm not sure why you think more CPU cores would help your build system initialize faster. I'd get a modern i7 not because of the number of cores, but because of the cache sizes, or if you prefer AMD's architecture, the same for FX processor. It is a mix of raw cycles so a high clock rate, and also cache size. A cache miss is equal to about 50 CPU cycles, so if you can avoid cache misses with a larger cache it pays off quite well. Since the chip manufacturers understand this (more than most consumers) the high-end chips have both more cycles and more cache, and they cluster their offerings based on the combination of raw cycles and cache.
That is true if you're making a build machine, and even more important if you're making a machine to play high-end games. Basically get the highest numbers you can reasonably afford.
- memory, 8 GB or 16 GB? Maybe 8GB but faster? Overall, should I bother with memory speed? Is it a bottleneck of any sort?
16 GB of fast memory. YES it is a performance bottleneck, but less of a concern in a build machine. If you're also going to be playing high-end games, you'll want fast memory.
Something you did not mention is your motherboard.
They make a difference to. Not in computation and memory, the integrated memory controllers take care of that, but a modern PC is made up of more than just the CPU and memory chips. Motherboards these days have fully integrated Audio, LAN, USB, disk controllers, etc. Some people buy cheap motherboards that have crappy disk controllers then wonder why their high-end disk drives are sluggish. It won't need to be the ultimate high end board, but it can give a 5% or so performance difference in general, with variations between audio quality, USB performance, and the rest.
If you're planning on overclocking, your motherboard selection will also make a difference in how far you can push the box.
With all that in mind, none of it is REQUIRED. You can build and develop games on just about any stock off-the-shelf commodity PC. The $100 special at WalMart isn't the best computer in the world, but it better than you could find five years ago, and is more than enough for anything an individual will create.
