Since you're using EasyTune5 I'll assume you have a DS/S/DQ board.
When you start your computer hit Delete to enter the CMOS Setup menu (What a lot of people mistakenly call the BIOS). Now scroll down to Advanced options and disable C1E, E1ST, SpeedStep, Thermal Monitoring. Now hit Escape and go down to PC Health. Go down to Fan Control Method and select Disable, this allows the fan to run at 100% all the time. Providing extra cooling.
If you don't have an aftermarket cooler already you might consider the AC 7 Freezer it's one of the cheapest and easiest forms of air cooling for only $25.
Now you'll want to backup and go into the MIT menu at the bottom left. Go ahead and enable Clock Speed Control. Should be the first option. Now you'll see that 266 is no longer greyed out. Since you've been to 2.3Ghz go ahead and change that 266 to a 350. Now go down and change the Memory Multiplier from Auto to 2.0 I assume you're using DDR2 800 Mhz here. If that's the case you can go all the way to 400.
Also you can leave the PCIe Frequency on Auto or lock it @ 100Mhz
At the bottom you will see voltage controls. If 400 is not happening you'll need to increase that bottom one from 1.325v to something like 1.35v all the way up to 1.4v if you start to hit 450+
Backup for a second now to the main menu and hit Ctrl-F1. The screen should blink and reset the cursor. Go back into the MIT and you should see some extra options. These are memory timings and are there for later, once you hit a good speed you're happy with see what your auto timings are and try to go down from there.
Get these two programs for now. Remember temperatures over 60Cº and voltages over 1.4v can become dangerous.
Also be sure to never over compensate on voltages. Move up a little at a time. If you need more help post more questions here. We'll be glad to help.