Before you go tearing that thing apart, what are the ambient room and case temperatures? I thought the 4800-series cards were supposed to downclock themselves at idle, reducing the core clock and voltage... I'm unsure if that would be controlled by the driver or the card's BIOS. What version of the Catalyst Driver Suite do you have installed?
Should i replace the thermal compound with arctic silver 5?
Should i add a 120mm case fan to the side of my antec 300?
Or should i replace the GPU fan altogether?
Please give my some pointer.
Look at case temps and the airflow around this card. Seeing that it doesn't vent to the outside the heat can build up so you want to get it out faster.
My previous setup had a fan on a stick (Antec made I think) pointed over the card with an extractor fan below the card. That created a "fresh" airflow around the card. Nonetheless, don't be surprised at idle at max temps - just make sure that there's cool air getting to the gpu.
Ideally, air pulled in from the front bottom should provide this and then a extracting fan top rear should get rid of this (of the case).