I'm surprised, I figured my card was substantially less powerful than the ones I mentioned above. But you're saying that's not the case? A dumb question, but how do I see that? I mean one is even of the same brand, and the numbers are bigger (Asus Radeon HD 6450 1024MB Silent), twice the memory and the model name 6450 suggests (well, I thought at least) a more powerful card than my 4850.
There is no hard and fast rule to know which card is faster just by looking at the model/name or amount of RAM, the only way to really know is to check in sites such as techpowerup, and compare the models you are interested in. It is key that you check the games you use.
Techpowerup seems to really care about GPU noise, that's why I tend to take a look at it when it comes to GPUs.
For performance comparison Google is your friend: you might want to search for "6450 vs 4500 ATI", "5450 vs 6450" and the like for getting hard data.
If you don't play a lot of games and the only problem with your current graphics card is the noise you could try to make it less noisy instead of buying a new card. Slow down the fans or even remove them. Idle temps should be ok without the fans, and depending on case ventilation you might even be fine while gaming. TDP for the HD 4850 is 110W I think. Removing the fans and replace with a slower more quiet 120mm fan is another option.
Removing fans in a 4850 could end up in a disaster, I would not even consider that. And using a big slow fan without a good heatsink (think of Accelero S1) can be dangerous, and even if the GPU temperature were ok, VRMs might get too hot.
Maybe he can change the fan speed as you suggest, so that the fans do not run that fast when temps are low. That might help to make noise bearable while not gaming.
Besides, he might try underclocking and undervolting the card, so that it generates less heat and the fans do not need to run so fast -if he finds performance is good enough.