5750 is faster than a 4850. Mobile or not.
not always
10 years ago higher model number meant faster. these days ATI and Nvidia make 10 different SKU's in the same generation with some "current" generation SKU's being slower than previous generation cards. reason is that computer makers want a current generation model number at a cheap price.
Apple does the same thing as Dell/HP. sell a 5000 series graphics option where the silicon is not very fast. you have to look up the model on ATI's site and read the description to see what you're buying. last month when my employer bought me a Lenovo laptop i spent 2 hours doing this trying to pick the best one for the budget