New build. Budget $750USD

I think my PC is worsening. It used to be very fast and I understand that computer hardware degrades over time. I also tried to defragment my hard disk and memory. However, I seem to play more graphic intensive games and other softwares like Adobe Flash CS6, and lots of downloads. I bought a Sapphire Radeon HD 6670 (Low Profile) 1GB GDDR5 graphics card and recently a stick of 2GB DDR3 RAM, which, indeed gave a huge boost in performance in gaming. I used to lag even on lowest settings on some games and I hosted a server to game with my friend with 2GB of RAM and both of us were lagging so much that we cannot play. But now, no lags in games, smooth gaming on servers. I also watch HD Youtube videos. It loaded very quickly when it was quite new, but now it took much more time to do that simple task but thanks to the RAM and graphics upgrade, it stopped worsening. But I know this cannot last long, so I decided to build a new computer.

I have a few requirements for hardware:
CPU : Intel Quad core 3.1GHz or above, the newer technology the better
MB : Any compatible with CPU and support OC and onboard graphics and PC3-10600 1333Mhz RAM mATX or ITX 4-pin 12v CPU power plug
HDD : Any SATA >250GB
Optical Drive: Any SATA DVD Burner

Budget $750

I already have the PSU and Case and 1x8GB DDR3 (PC3-10600)RAM. Check if it is compatible. Give me a good suggestion for graphics card if the onboard graphics is not good enough.I may do some overclocking whenever I think my computer is slow so I chose Z68 chipset.

Here is what I have chosen
CPU : Intel i5 2500k
MB : MSI Z68MA-G45(B3)
FAN : Cooler Master Vortex Plus
HDD : WD Blue 500GB WD5000AAKX
Optical Drive : LG GH 24NS90

****It may be over budget***
Also please check if there are better options or itx form factor MBs that suit my needs