How is a $1400-$1500 price with the same gpu (675mx) or one step better(7970m) a better deal than a $900 laptop with a 675mx?
The OP lists a 675mx, then mentions a 675m... be careful, the 675mx is a step and a half better than the 675m. If the laptop has a 675mx, its an incredible deal ($500 off). If its a 675m, its an OK deal. (about $100-200 off)
The 675mx is an underclocked Desktop 660, with slower GDDR5 on 256-bit memory interface. Early benchmarks place it at 4500P in 3dmark11. (it will perform between the desktop 650ti and 660.)
960 cores @600MHz (48bil textures/sec)
256-bit GDDR5 (115.2 GB/sec)
960 cores @980MHz (78.4bil textures/sec)
192-bit GDDR5 (144.2GB/sec)
768 cores @928MHz (59.2bil textures/sec)
128-bit GDDR5 (86.4 GB/sec)
The 675m is a rebranded 580m... thus it's an underclocked 560Ti. About 3600P 3dmark11.
As for what the laptop can do, the one with the 675mx (the better one) is roughly the same (within 10%) as a desktop i5/560Ti. There won't be anything you cannot play, even at 1080p. You might just have to set it to ultra, but drop the antialiasing to 2x instead of 4x... The only exception will be machine-munching games like Witcher 2 or Metro 2033.
If its the 675mx, I'd get it without hesitation. Worst case resell it for more.
If its the 675m, I'd counter-offer a bit lower.
Yes, you could build a better desktop... but don't forget you'd have to count a screen.
(which the laptop includes)