Videocard numbers never seem to make sense to me. I want to buy Dead Rising 2 for PC but I'm not sure if I meet the minimum requirements. I have a ATI Mobility Radeon HD 4670 and yougamers.com tells me it doesn't meet the minimum requirement which is the HD 3850. I've read that the performance of my videocard lies between the destop HD 4650 and  4670. It would make sense that the HD 4650 is better than the HD 3850. Doesn't it?  So I should be able to run Dead Rising 2?

With video cards, the first number signifies what series it is from, and the following numbers determine how high-end that card is within that series (generally anything below 60 or 600 is a low-end, "budget" card). A high-end card from the HD 4000 series will always outperform a low-end card from the HD 5000 series.
 
I'm not 100% certain, but a (desktop) 4850 is solidly better than a (desktop) 5670, so I wouldn't be surprised if that were the case for 3850 and 4670 as well. 
 
Also, hardware requirements scanners can go haywire with notebook video cards thrown into the mix. In some cases they will say you can't play the game in question when in reality it will run well. In other cases they can also overestimate how well a laptop's GPU can perform.
 
Here's a video of someone playing DR2 with a desktop 4670; can't find any firsthand experience from someone who has played with a Mobility HD 4670.

yep as bigandtasty states , you can only compare numbers within a series , not across different series.
 
when it comes to desktop cards , 3850 is about 11-17 % better than the 4650.