You can fit a Go7900GTX in the E1705
However, I'm not sure about the new 7950GTX
There's some minor "modding" to make the GTX fit in the E1705, but nothing serious, just removing a few bits of plastic to make it fit.
The 7900GS's actually overclock quite well. I have mine running at faster than GTX speeds. (I think it lacks a few pipes compared to the GTX, also some RAM...but the performance is still pretty good.) I score right at 5000 on 3DMark06.
I'd recommend getting as fast of a CPU as you can afford, I bought mine before the C2D's were out, so I have a Core Duo 1.83Ghz. It's not bad, but I'd sure love to have a C2D 2.33Ghz.
But I really recommend the Dell Inspirons. They are a great deal, you get pretty good performance for a low price. (Mine was $1530 shipped, , and you can now get one with a C2D (Merom) for around the same price.
But like Nik said, notebookforums.com or Notebookreview.com (Forums) are great places to get all the answers.