imo your cpu and gfx card both are bottlenecks for 2011 games. the ddr3 ram on the 6450 doesn't help either. try turning game settings to lowest e.g. 640x480 or 800x600 with direct x 9.0. you can monitor cpu and gpu temps during gaming with gpu-z and hwmonitor. clean inside the case, clean the cpu hsf using compressed air if it's dirty.
i changed my mobo twice...do u think its the problem of motherboard? the idle system temperature is 45 degree and cpu idle temperature is 60-75 degree when i play games cpu temperature goes around 90 degree
could it be that you're using the stock cooler? the idle and load temps are really high. the games are cpu and gpu intensive, so they can drive temps high. iirc cpus should operate at mid 60s (on load) for optimal operation. i think your cpu is throttling down after getting too hot during gaming, that's why you lose fps after a while. you could try changing the cooler, but you could be better off with a new pc if you got money to spend on a new system.
The Pentium D does indeed run hot, but it shouldn't get up to 90 celsius on a gaming load. Your CPU is definitely throttling to avoid damage. Odds are on a system that old the thermal paste has degraded on the CPU, so the cooler isn't working as well as it should. You'll have to remove the cooler, clean off the CPU and cooler, and apply new thermal paste, that should fix the heat issues.
Having said that, I would definitely consider a new system. The Pentium D is really slow by today's standards, and the 6450 is not really a gaming card. You won't get good performance out of either. If you are looking for a cheaper upgrade option, you could check your board's CPU support list and see if it will support a Core 2 Duo or Core 2 Quad. A new CPU would definitely help performance, but don't pay too much for it, a lot of the LGA775 CPUs sell for ridiculously inflated prices, to the point where you might as well build a new system.
A new graphics card is also in order, but most cards more powerful than the 6450 will be held back by the Pentium D. You may also need a new Power Supply if your computer has a really weak PSU, like 250 Watts. Something in the 400 Watt range minimum is needed for most decent gaming cards.
That will definitely be a huge improvement over your current CPU. You're probably going to have to get new RAM as well, unless you're getting an AM3 board that supports DDR2. Be aware that there will be no upgrade path beyond the Phenom II X4 going this route. AM3 doesn't support anything newer than that, not that it makes a difference, the Bulldozer CPUs are at best equal to the Phenom IIs.
A new graphics card would also definitely help, but you can upgrade that sometime later if money is tight right now, a Phenom II system is still a good foundation to build on, much better than that old Pentium D. That Phenom II will handle any single graphics card without much trouble.
i am planning on buying amd phenom II x4 965 black edition for 6680rs and a am3 motherboard
if you're getting an amd ph ii x4 965 be, get an am3+ motherboard if you can afford one. the new 9xx chipsets support more pcie 2 lanes, newer amd cpus, are more futureproof. iirc 990x and 990fx (40 pcie 2.0 lanes supporting dual x16 mode) chipsets lets users use both cfx and sli. some 970 chipset motherboards have cfx/sli support as well.
make sure the 965 is available at your location. get a good cpu cooler for the cpu. lastly, get a good 80+ or higher psu with good amps on the +12v rail.