I can't provide the BEST explanation since I'm pretty new to OC'ing myself but the basic process works like this.
1) Download the following programs: prime95, linx, hwmonitor
2) Reboot your computer
3) Hit the del, f12, or whatever key enters your BIOS
4) Go to FSB speed and change it to e.g. 10% higher than what it is now. I'm not sure, but I'm guessing the clock multiplier is locked on the E7500 so you can only change the bus speed to increase overall clock speed. Leave voltage as is and save changes and run prime95 stress test and linx (20 runs) for a while. If temps stay where you are comfortable, then you're good and can try to increase the bus speed again.
5) Now you have 2 possibilities: a) everything works fine so you're good b) you get a BSOD. There are 2 ways you can combat the BSOD. Take down your bus speed or increase your voltage. Take small steps though. Remember, a 1.4 volt processor usually only needs to go to like 1.6 or maybe 1.7 MAX before it's ready to start cooking really hot! Steps of 0.1 volt are probably enough.
6) Rinse and repeat volt tweaks until your system passes prime95 stress test for at least an hour or two and you will be fine. If the system can not bsod AND temps stay low, you're golden!