Well, there are so many factors. If the room in which your computer is situated is warm, then no matter what, your CPU temps will be warmer than someone with a computer in an air-conditioned room. Also, what case you have, how many fans, etc, also make a difference. I dont have any experience with the Sonata at all, but if its anything like Antec's other cases, its pretty nice. How many fans do you have in there?
If it were me, I wouldn't worry about the temp. 51 C isn't terrible, especially if the room is warm. But bringing the temp down would give you peace of mind and more headroom in case it gets any hotter. I'm not up to snuff on P4 heatsinks, but I know the Alpha 8492 is a great cooler. Pair that with a YS Tech variable speed fan, and you have top-notch cooling, and you can dial the speed down on the fan if you want quiet.