Need help on Overclocking

[pochama]

CPU: E7400 Dual Core 2.8Ghz
RAM: 2X 2GB RAM with Heatpipe cooling 800Mhz
MOTHERBOARD: Asrock P45R2000
GRAPHIC CARD: ATI 5870
PSU: Corsair 500W VX
CPU cooler: http://www.thermaltakeusa.com/Product.aspx?C=1148&ID=1553

Software to Overclock: Asrock OC Tuner

How much should I increase up to?
Do I have to bother about voltage, RAM, PCI-E?
Should I overclock Video card as well?

Which CPU works faster on directx 11 games?

E7400 (stock) or E7400 (OC) or Q8300 (stock or OC)

 

[jsc]

Windows based overclocking utilities do not work that well. Learn to use the BIOS.

This should be your first stop.
HOWTO: Overclock C2Q (Quads) and C2D (Duals) - Guide v1.6.1
http://www.tomshardware.com/forum/240001-29-howto-overclock-quads-duals-guide

Next stop should be a guide for your particular motherboard. Google is your friend.

Which CPU works faster on directx 11 games?

E7400 (stock) or E7400 (OC) or Q8300 (stock or OC)

Obviously, an OC'd E7400 will work better than an E7400 running at stock freqs. The Q8200 CPU family is not known for being great overclockers. You should be able to push the E7400 much higher. The only thing that might make a difference here is if the games that you are interested in support quad cores CPU's