Alienware M17X R2 upgrade options

Hello folks!!!
I bought an m17x r2 off ebay for real cheap . I'm happy to say that I have officially joint AW club!!!!
I didn't receive the unit yet , I paid few days back .
I got 300bucks to spend for a decent upgrade.
I bought this in the intention to fullfill my daily gaming needs.

It's current specs:
CPU: i7 720qm
GPU: gtx 285m (x1)
Ram: single 8gb
Hdd: 500gb

I don't need a ram upgrade for now.
I have a extra Kingston 250gb ssd hanging around me so I could use it for hdd upgrade.
I did some research on the upgrade options for the CPU and gpu.
And I came up with these :
CPU: i7 920xm
GPU: gtx 680 or dual hd 7970's

I read other threads about successfully installing dual hd 7970's in r2, I feel it's a good route to go.
Then I read about gtx 680m , it seems to work on this lappy but sli option was not explained or tested more precisely . I read on a old thread on a guy attempting to run dual gtx 680m but in the half way he stopped and didn't update the thread with results.

Then I read about gtx 780m upgrade , he did put forward a huge attempt , with the results of getting the gpu working on external screen but not on it's own screen. Can it be because it was non dell card, did anyone test with dell card ?

Guys I need your advice on what route to go for max performance for the $ I pay .

Thanks for your advices , I really appreciate it.

Edit: I would also like to know where can I get these gpu's from ? Ebay or nbr marketplace , it doesn't mind even it's a used one.
Also I would need a power brick for this , should i get it through ebay or if anyone has one extra feel free to send me a pm so we can discuss.

I don't like ebay much because of cheap Chinese stuff and very bad people these days , I got ripped off two times on desktop gpu's.

 

I am researching on this as well currently, as you said the 7970's in Xfire are a known quantity but read carefully as there is some modding involved if you run in SLI. One card seems to be fine with the correct BIOS version. The toss-up as I have read is that the 680m delivers more consistent frame rates while the 7970 delivers slightly higher frame rates but with less consistency.

To date I have heard nothing but good reports on UpgradeMonkey, heck they will even flash the BIOS apparently. The 920xm seems to be a no-brainer for the money.

 

Yea , I see that too : so I guess it's better to stick with one gtx 680.

Yup that's correct.

I recently saw a ebay auction : a guy selling a m17x r2 with gtx 980m , I was shocked when I saw it.
I don't know if he is a real seller . But he did post a picture of m17x r2 .