A 9800GT is more powerful than a 240. The GT 250 is the newer model of the 9800.
The 3 levels of speed in your price range are
basically its a 50% speed jump per level so you will have to check price/performance wise where you live. The mid tier is probably the best balance for your CPU. I also listed them in my preference order (left is most preferable) due to features like DX11, heat and power use.
You should also look at overclocking that CPU to get a 10-15% speed increase.
You may also need to get a new power supply to handle a new graphics card and you did not list your power supply so my bet is you will.