Originally posted by: tim924
Expect half the performance of a desktop video card for the similar model(9600m Vs 9600GT,or 9800m gts Vs 9800GT etc.)
That's a little much. You will generally get better performance than that. Keep in mind the fact that notebooks typically use much slower processors which often brings down benchmark scores when comparing to desktops.
I have a 9800M GT running on an E8500 (desktop CPU) at 3.16GHz and get about 10200 in 3DMark06 at all default settings using Windows XP. That's roughly equivalent to 88% of what the desktop 8800GT (9800GT) card would get with the exact same CPU. (Based on objective personal testing with multiple systems and factoring about a 3.5% performance hit using Vista)
Of course this rule only applies to the 9800M GT which is a very hard to find mobile card but equivalent to the 8800M GTX. The more common 9800M GTS is actually a faster mobile card but with fewer shader processors and consistently benchmarks quite a bit lower than the mobile GT so it would in fact be much slower than the 8800GTS desktop card which is faster than the desktop 8800GT (There is no 9800GTS desktop card to compare it to). You can thank NVidia and their crazy and inconsistent naming schemes for this.
On the other hand the top of the line NVidia mobile card, the 9800M GTX is the same card as the GT running at the same speeds but with double the memory (1024MB) and more shader processors and tends to be about 15% faster than the GT. This puts it very close in performance to the desktop variant of the 9800GTX (512MB). While I haven't had the chance to personally benchmark either of these cards they are probably within about 5% of each other based on what I've seen from others.
So there you have it....there is no hard and fast rule to compare the NVidia mobile lineup to the desktop lineup using the very confusing NVidia naming. I don't know about the mobile AMD parts.