^ what tea urchin said. If you are planning of going multi gpu build, make sure that you have a decent monitor to get the use of them extra frame rates. 120 or 144hz monitor is a must in accordance to your plans.
In terms of a pc case, well rampage x99 is a full atx mobo, therefore a midi tower would not necessarily fit it inside by default, so you're looking at a full high tower here. When it comes to these babies, what I've learnt and seen in general is that you rarely get them on a cheap, and higher price (within reason, of course, I've seen some slick looking aluminium cases or full glass towers and heck knows what else for hundreds of dollars) generally means better build quality, so it would be safe to assume whatever you go for will be a quality product. I won't even start naming examples as they are so subjective to taste and what you'd prefer. Few obvious pointers (or one main pointer to be precise) would be to make sure you've enough space for fans and cooling equipment, to guarantee decent air flow ecen with two or three gpus stuffed in there or in case you want to go custom water Cooling one day (which is not a necessity anymore due to how silent and effective aftermarket gpu coolers/custom cpu coolers are).
Cpu, a big choice that goes next, you have three options (as of now). i7 5820k, 5930k or 5960x. As with all previous 60x'es Intel has released, gaming wise you wouldn't see much improvement, nor longevity, over 30k model. Plus 60x costs twice as much. 820 models, I've noticed as I've got a 3820 myself, are weird in the sense that they have all the specs on paper topping those of 3770k (or any newer 770k of equivalent generation) but don't surpass and barely match performance in gaming. So my recommendation would be 5930k.
Ram. Ddr4 is still quite expensive, so at the end of the day its your choice (or the size of your pocket that is) what you want or can go for. I would say ddr3 is still more than viable, and win x99 mobo you will always have an option to upgrade. 8gb would be a bare minimum, just about enough for casual use (and gaming with best presets) but you would probably want some elbow room, so look at double that number.
Graphics cards. The forever going debate of nvidia vs amd. And unlike cpu market, when it comes to high end gear, amd has some powerful stuff (and great vaule money wise) to offer. I'm waiting for my r9 295x2 to arrive which I bought for ridiculous price on black Friday sales, that card alone stands as a strong enough proof of wkat I've said earlier. Looking purely from theoretical perspective, Hawaii architecture is getting dated (more than a year in this day and age in tech world is aged, I'm sure you'll agree), then again maxwell (nvidia counterpart) was introduced early this year, and we see glorious results in performance (and price, esp in gtx970) with their new 970/980s. So from that angle nvidia has some advantage. But in terms of price, 280x and especially 290 are on record low, which are still awesome cards on their own right. So once again it comes down to whether you have or want to spend extra here. All choices, amd, nvidia or just sticking to what you have and upgrading/adding extra gpus later are very viable.
Now to go back to one of your questions, I would (if you can gather from what I've said so far) say build your pc around the parts you've got from scratch and anew. You will boost ots longevity by a good margin in doing so.
One last advice. Good psu (=good quality) is by default a must. Xfx, superflower, Silverstone, also likely corsair are among the worthy choices. From the top of my head you'd be looking at 8-850watts on gold plus, and 1000w or more for a dual gpu setup (1200+for three).all in all, I'd say this little project would set you back by 17-1800$. That's just an estimate considering all the parts and my best guesses.
Hope this helped
and last but not least- congratulations on winning the competition! What an awesome early Xmas gift! ^____^
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