ya maziar is right. but just for ur info, the 5850 in the GX640 is by name a 5850 but infact its much better.. its more like a slightly lowerclocked 5870m (or a very high clocked 5850), and because of this labelled a 5850 and not a 5870. if u look at the benchmarks, u will be surprised to see how low the difference in benchmarks actually is.. infact i will give u the comparison right here and now..
http/www.newegg.com/Product/Product.aspx?Item=N82E16834115792
BATTLEFIELD BAD COMPANY 2, 1920X1080 RESOLUTION, HIGH SETTINGS, HBAO ON, 8XAF 4XAA
FPS of GX640 : 30.8
FPS of notebook with 5870 and core i7 720qm: between 30.8 and 31.8 (taken from different notebooks)
COD Modernwarefare 2, 1920x1080, high settings
FPS of GX640: 46.7
FPS of 5870 with i7 720qm: between 48.7 and 51.1
DIRT 2: 1920x1080 ultra preset AF, 4x AA
FPS of GX640 :31.8
FPS of 5870 with i7 720qm: 29.5 to 33.8
as u can see, there barely any different. just a few FPS, at most 3 to 4 FPS.. not worth extra money i would say.