The Antec 300 is wide enough for any cooler that I know of.
The best would probably be the prolimatech megahalems rev b.
But, your current cooler is decent, and I would not expect any magical increase.
Here are some things to check:
1) Is your cooler mounted well? Are all the mounting pins completely through the motherboard and locked?
2) Did you use too much thermal paste? It will act as an insulator if there is too much.
3) Can you add some intake fans to the case to increase the airflow?
4) What kind of cooling does your graphics card have? A double slot direct exhaust cooler is best because it sends the hot vga air directly out the back. Other coolers recirculate the hot air, increasing both the vga AND the cpu temperatures.
5) 3.5 is not a bad OC. Some samples simply won't go much higher. How much value will you get from higher OC? Reduce the oc to 3.0 and see how much it impacts your FPS while gaming.
If you do not get much of a decrease, it is pointless to reach for a higher OC.