|"I have a fan on rear side"|
Is it in the rear or the side? If it's in the rear, it should be an exhaust fan, in other words, it should blow out of the case, not into the case. If it's on the side panel, remove it & install it in the rear.
"it was done by professionals"
If your CPU temp is hitting 70-80C, it's because of a problem with the CPU heatsink. I don't care if it was installed "by professionals" or not. Could it be that you knocked into it during your monthly cleaning? Hopefully you use compressed air to blow the dust out & not a vacuum? NEVER use a vacuum due to the static that can buildup in the hose then discharge into the case. Your AMD CPU uses the "middle dot" method of thermal paste application. You might want to remove the heatsink, remove the old thermal material, then apply a fresh dab of paste. Here's how it should be done:
"already set the settings on Medium and don't want it to lower it"
Your system will never play games well as long as you use the weak Radeon HD integrated graphics (aka IGP). Changing the video memory setting will NOT improve gaming significantly because memory isn't the problem, the IGP is the problem. Like I said, you need a decent video card. If you don't want to install a card (or can't afford one), your only choice is to lower the game settings, especially the gaming resolution. I don't mean the monitor resolution, I mean the resolution in the game settings. If you insist on trying a different video memory setting, enter the BIOS & find UMA Frame Buffer Size. The choices are [Auto] [32MB] [64MB] [128MB] [256MB] [512MB] or [1GB]. You can also try overclocking the IGP. But don't expect miracles.
"Power Supply : Odessey 450W"
If you decide to install a gaming video card, the power supply will have to be upgraded. Odyssey is low end garbage. I'm surprised it hasn't given you any problems yet. I'll never understand why decent brand name power supplies are so difficult to find in India or is it that you just don't want to spend the money? 500 INR (less than $10 US) for a 450W PSU/SMPS? Do you REALLY think it's a quality unit at that price?