Well im not to sure about the different performances but the processors clock speed makes hardly any difference at all but the laptop you looked at only has 2mb cache as opposed to 4mb so may make a bit of difference but mostly only in more modern games, though im no expert.
I dont know too much about the 8400m but it looks like it is 128mb dedicated so thats okay for gaming you may have to lower the textures though. From this website
clicky its clock speeds arnt greatly less than the 8600gt (could be overclockable as newer laptops can take a we bit of it but id check some owners forums for the Asus before you try anything) and its 16x shared pixel/vertex shader pipelines (against 32 for 8600) will still be good for RO. !*Edit*! the one in the ASUS a 8400m G only has 8 pipelines but have a look on the link above to check the difference.
For reference my old ati 9700m 128mb was fine for textures etc but having only 8 shader pipe lines it suffered with all the fancy shaders in dx 9 but the 8400m GT should handle them much better.
As well as looking at notebook.net look at the forums on
notebook review as you will probably find some people who have got the laptop and will have posted any problems or praises that they have with it.
Oh and tubocache is alot of crap and hardly makes any difference with actual game performace but makes it boot quicker..maybe...sometimes...
Another thing to think about is that some laptops have just had a little price drop since the summer so if your in no rush you would probably be able to get a bargain after christmas.
Edit: Im not to sure about your desktop speeds but i would imagine that the ASUS laptop would be a good replacment. Maybe a bit slower in older games but faster in newer games, though I know relativly nothing about desktop specs from the geforce 6 era
