and i think i would get a lot of performance of my gpu by overclocking. I were able to overclock it from 650 to 675 mhz... so not really much. its running normally in about 40 Celsius. And when i run MSI kombuster its around 60 Celsius
But what are the rest of your machines specifications?
You've purchased a machine from a company that mass produces. They usually use OEM Components. Which can be low grade; thus the reason the prices are usually appealing to consumers. The next dilemma; they figure out the lowest cost they can, where they can cut corners.
An example would be your power-supply; if your current machine requires 350 watts to run. They essentially will give you 350 or 400 watt power-supply. So you have little wiggle room; so purchasing a higher end GPU may require an additional Power-Supply.
That's why it's important to know all of your existing parts.
Also what was stated above by Geo is also true; your essentially testing to see if your games are CPU reliant or GPU reliant. Boosting your CPU in a game that requires more GPU power; won't benefit nor would it be an upgrade and vice versa.