Need help upgrading my Alienware Aurora

I ordered my Aurora machine back in May of 2010, and while it's been very good to me (only 1 video card death) through a solid year and 4 months of logged gaming time...I am now coming to the realization that it's time to upgrade the key components in order to keep those graphics sliders up with soon to be released expansions for the games that I enjoy.

Please note that my tech knowledge is quite dated.  I took courses on computer repair 13 years ago, when the insides of computers were very basic. Fast forward to 2014, and I'm sitting here overwhelmed by the amount of information I need to obtain before I am certain that what I *would like to* upgrade is compatible with what I *can* upgrade.  This is where I turn to you AC community members for help.

 

What I know about my system

Motherboard: 

Alienware (proprietary?) 04VWF2  (A01)   Intel X58 chipset.

*board is visually ID'd as "E203413 DE0907"

PCI Express x16 graphics interface

Bios version updated to A11.

Processor:

Intel core i7 920 @ 2.66GHz (8MB cache)

*Alienware High-Performance Liquid Cooling*

GPU:

Radeon HD 5670 (crossfire setup) *these MUST go if nothing else!

1024 GDDR5 Memory

 

RAM:

6GB Triple Channel 1066 DDR3 @ 1333MHz

What I would like to upgrade

GPU/s:

Even back in 2010, the HD Radeon 5670 cards were barely mid-level in performance, but were among the cheapest in the range of choices Alienware offered with their customized Aurora system configurations. 

As I previously mentioned, one of the cards died a cruel and sudden death with the only pre-warning signs coming in the form of split second fast black lines randomly flickering horizontally while playing World of Warcraft: Mists of Pandaria.  My girlfriend ordered an identical system and was generous enough to downgrade her system to 1 GPU and give me one of hers (the one I saved from what I thought would be certain coffee spill death, with rubbing alcohol and baby wipes...no joke!). 

Now those same symptoms, I think, are slowly showing up again as I've observed a few instances of those same black flickering lines in the same game this week.  So these cards are at the top of the upgrade list.

Potential upgrade:

I have my eye set on the Geforce GTX 750 Ti  EVGA Overclockedversion - http://www.newegg.com/Product/Product.aspx?Item=N82E16814487024

I want to dump ATI completely and go back to NVIDIA due to my dissatisfaction with the last 10 or so driver updates, including beta drivers from ATI.  I also want to drop to one powerful GPU instead of crossfire or SLI. The games I play (Blizzard games) do not officially support multi-gpu setups anyways.

I have no idea if this card will work in my motherboard.. are PCI Express 3.0 cards compatible with the x16 graphics interface?

RAM:

This is embarrassing because upgrading RAM is supposed to be simple.  Not with the only other experience I had in attempting this with my last computer.  After 2 trips back to Fry's to exchange the wrong memory types I bought.

Potential Upgrade:

I want to make sure I do it right this time around.  I just want to go from my 6GB to a 12GB total upgrade. I don't need any more than that, and I probably don't need more than 6GB..but I want to..just because.

Should I just get another 6GB of 1066, 1333MHz to go with the 6GB that came with this computer?  I have no idea what Alienware did with the RAM that is in the machine currently, as far as overlocked or not..does that matter?  Does the manufacturer of the RAM matter if they are both different?  I assume the sticks that came with the computer are not name-brand.

 

Processor:

By now, the reader is fully aware that I am shamefully stuck in the distant past with regards to how computer technology works..and this is the most confusing of the components I want to upgrade, for various reasons.

I listed my motherboard specs to the fullest of my researched knowledge in the hopes that someone could help me understand some things I've been reading.  I found a link, through these forums, that led me to a compatibility chard for motherboards and processors.  I have no idea how old that chart is, but what I got from reading it is that I am limited to a small choice of upgrades due to both A) having an Alienware proprietary board, and B) the chipset on the board being X58. 

Is that correct? And, if that is correct, then I am limited to.....

Potential Upgrade:

Intel Core i7 990x? - http://www.newegg.com/Product/Product.aspx?Item=N82E16819115079

*note that if you click on the link you will find that the product is out of stock..as are ALL of the 9xx series processors that the compatibility chart shows as being compatible with my motherboard.  None of the big computer part retailers have them in stock..leading me to believe that these processors are obsolete..?

Am I really stuck with the limited choice on processor upgrades?  And where else can I find an i7 990x without having to deal with the likes of EBAY if so?

I'd like to do all of the upgrades without having to swap out the motherboard or there is no point..I might as well submit to buying a brand new computer.

I would very much appreciate any help with all of this and offer my thanks in advance.