What are the dimensions for the stock fan? I think it's 80mm, but I'm not sure.
If you're trying to get your processor cooler, and have less fan noise, you'll need a new heatsink and fan. Just purchasing a new fan will probably allow you to make less noise, but at the same time will raise your processor temperature, so you'll need a more efficient heatsink to do both jobs at the same time.
I think this may be what I'm looking for - the OCZ vanquisher. Either that or it's the Arctic Freezer 64. The second one seems to have a lower airflow but seems to have a better heatsink... I'll have to look into it.