+1.  32BG RAM is the way to go.  For processors, I'd personally go with the 4790K.  A big factor for me is the TDP at 88W opposed to 140W for the 6 core.  Also, your base speed on the 4790K is higher, so it'll be faster for other things that don't utilize the multiple cores as well.

The GTX980 Ti would be a dream card for me.  If you can get it, I say go for it even if only for bragging rights.  If you could get one and compromised on a GTX970, you'll probably feel bad about it later.

I upgraded from a GTX760 to a GTX670SC, which is on the same level, but a couple of generations older than the 970.  I'm ok with it, because it was such a good deal.  But I drool every time I see a 980.

The SSDs will make a big difference too.  I have the game on an SSD and I can't tell any difference in real world performance between that and a RAMDisk.  

Whichever way you go, it looks like you're building a monster.  Pics when you do please.

Again, go heavy on the RAM. In a lappy, I'd actually advise avoiding an overly hefty processor, as it'll overheat the thing (plus driving your cost up by a couple of hundred quid). With an SSD and a lot of memory, an i5 is more than capable for 4Xes. Only if you want to play shooters or flight/space sims on it as well is it's probably worth the extra grunt.

Try to avoid a video card that borrows system RAM. 

http://www.amazon.co.uk/Asus-ROG-G751JY-DB73X-HID3-i7-4870HQ-2-5-3-7GHz/dp/B00ZZI4P1C/ref=pd_sim_sbs_147_4?ie=UTF8&refRID=1GDKWK8QPG5M5F2Q24ST&dpSrc=sims&preST=_AC_UL160_SR160%2C160_

How about the above ASUS? Forgive my ignorance but how do I know if the video card borrows system RAM??? 

Honestly, I've found prior-generation E3-based workstations to be excellent values.

Get something that has an E3, or E3 v2 chip in it, and it'll perform quite well.  They'll almost automatically have support for 32GB of RAM, and most will support 64GB.  They'll also have better than normal chipsets, which often means you have more than the standard PCI-E channels available for dual graphics cards.

The major issue is making sure you can get one with either a sufficiently large power supply (i.e. 600W), or one which DOESN'T use a non-standard PS and can be replaced with a generic PSU.

I have an older HP z210, with an E3-1240 (3.2Ghz 4-core/8-thread Sandy Bridge), 24GB of RAM (it takes 64GB), and an Nvidia 750 TI graphics card. It chows on GC3 with no problems, to very large galaxies.  Oh, and it was $500.  The major issue is that it's got a 400W power supply that isn't upgradable (damned HP). This rig also plays pretty much anything that isn't a super-hi-rez RPG/FPS  extremely well.  I play on a 27" screen, 2500x1400, with all the goodies turned on.

Seriously - look at the workstations on Ebay. Lots of very powerful stuff for $300-500.  Open-box stuff WITH WARRANTIES for under $700 in many instances. It's really quite a bargain, and all of it will play GC3 for the next decade.

I tend not to play the Super Abundant settings - I like a modestly spread out map where Life Support is important factor.

I've played Insane maps with 40 AI opponents, "Common" settings for everything, though about turn 600 or so. A bit under 14G for RAM consumption at that point for GC3.

If you're not playing FPS or RPGs heavily, you DON'T need the high-end graphics cards. The GTX 750 or Radeon 7870 are more than sufficient, and will be for quite some time.  That is, I suggest you spend no more than $150 on a graphics card now, and plan to upgrade in for a similar price point several years in the future IF  (and only IF) you're having lags on new games, rather than put down $300 on a higher-range card now.  It'll also keep your power and cooling budget down.

At 32GB RAM, mine doesn't seem to go to the paging file at all.  I have mine set static at 102400MB.

My GTX670 was DOA, so I'm stuck with my GTX760.  Still, even with only 2GB of Video RAM, it handles GC3 well enough.

I have a few FPSs and it does well on those too.  I'll need more card for my flight sims though.  

Looking through the couch cushions for more change...

Tech companies often use a 1:1 exchange rate. No one is entirely sure why.