Well, I hate to be the bearer of bad news.... but here goes..
The Clevo D900T motherboard has a NOTORIOUSLY high failure rate. A lot of people have come to the conclusion that it will do no good to pour any more money into a D900T-based laptop.
I had a Sager 9860 laptop with a D900T, and the motherboard failed multiple times in two years. In fact, this motherboard was SO BAD, that it drove my warranty service, ATS Warranty, out of business.
Your problem may not be your video card at all, it may be your motherboard.
If you are not actually having problems with your laptop, and you are just asking about a video card upgrade, then thank your lucky stars! You are one of the lucky few!
Alienware put out a D900T laptop, the Aurora m7700. Those who are under warranty, have been having a lot of issues, sometimes requiring replacing replacing multiple motherboards. Those who are no longer under warranty, Alienware will not even do an out-of-warranty repair on the 7700.
Seriously, if your D900T is still working well, DO NOT MESS WITH IT. Adding a 7950 GTX to it could cause it to have heat failures, etc.. Changing the motherboard for a vid card upgrade, could saddle you with a lemon motherboard, just like so many others.
By the way, the D900T will not accept a 7950 GTX card unless it is above a certain revision, which if you've had your laptop for a while, it won't.
I contacted Sager about getting the motherboard and vid card upgraded in my 9860. It will cost me $1000.00 + shipping both ways. And that's WITH a 10% discount for Clevo D900T customers.
At this point, you would be best served by getting a new laptop. Do not go with Sager, whatever you do. They sold the D900T model laptops through resellers, then abondoned any responsibility for a BAD DESIGN.
My D900T system is failing as we speak. I am going with an Alienware Aurora m9750 with dual 7950 GTX's.. They are on sale now, and you can get a pretty good SLI setup for about an even $3000.00.
Again, sorry to be the bearer of bad news.