asus p5g41t is an entry level motherboard. Althogh it supports overclocking, never increase cpu core voltage. It will increase heat dissipation and motherboard will be done. Instead of increasing core voltage increase bus speed. After increasing a couple of values of 10 or 20 boot the system and check system stability with intel burn test. repeat it until the maximum value is got. I'm using same board with cpu e6600 and bus speed at 325. I got a temperature value of 65 at maximum stress and 40 at idle. I use core temp for temperature readings.
You'll need a bit of electrical tape and either copper tape or a conductive ink pen. An Exacto knife and cutting board (can be found usually in arts & crafts section of stores) was very useful as well when cutting the electrical tape and copper tape during my experience. There's a lot of information about what works at the thread I linked to.
I used to own an e4700 which is very similar to your e4400. I did a BSEL 800fsb-1066fsb mod and ran my cpu at around 3.5 Ghz 24/7 using the 1.45v mod. Of course at that speed and voltage, an aftermarket cooler is an absolute MUST! I used the cooler master hyper tx3 and it was more than sufficient and very cheap. I'd recommend that or a cooler master hyper 212 series cooler.