I had a very similar system to yours before.
(Asus p5q-pro + E8500)
It was very easy to over clock and @4GHz is could even play Xenoblade decently.
(I used EU version tho, 25FPS, ran 100% almost always)
As far as I've understood most E8400 should achieve 4GHz too with sufficient cooling.
SInce you have good RAM (PC8500) you should be able to reach pretty decent speed too.
I even ran mine at 3.6Ghz for a year on stock cooling, still staying in range of pretty acceptable temps.
The E8000 series should be below 70C btw when running things like prime95.
There are tons of guides how to clock the E8000 series around the web.
(Asus p5q-pro + E8500)
It was very easy to over clock and @4GHz is could even play Xenoblade decently.
(I used EU version tho, 25FPS, ran 100% almost always)
As far as I've understood most E8400 should achieve 4GHz too with sufficient cooling.
SInce you have good RAM (PC8500) you should be able to reach pretty decent speed too.
I even ran mine at 3.6Ghz for a year on stock cooling, still staying in range of pretty acceptable temps.
The E8000 series should be below 70C btw when running things like prime95.
There are tons of guides how to clock the E8000 series around the web.
Desktop HTPC:
Gigabyte Z97X-Gaming5- i7-4790k MSI-B75-E33, i5-2500k
Corsair Vengeance 16GB RAM Crucial 8GB RAM
Gigabyte GTX780 GHz Edt. ASUS GTX960 Strix
Win 10 Pro / OS X 10.10 Win 10 Pro
Gigabyte Z97X-Gaming5- i7-4790k MSI-B75-E33, i5-2500k
Corsair Vengeance 16GB RAM Crucial 8GB RAM
Gigabyte GTX780 GHz Edt. ASUS GTX960 Strix
Win 10 Pro / OS X 10.10 Win 10 Pro