Today, a single core CPU like the sempron 140 is not meant for gaming. 1 core is simply no longer enough for a mainstream or even budget gaming system. All current games use 2+ cores. All more recent use 3+ and some use 4 cores.
If you can, an Athlon IIx3 is very worthwhile and the best budget gaming CPU.
That said, the sempron will work. Especially with a lower end video card. You will still be CPU limited but you will be able to get playable framerates in many games.
I just did a test with COD4 at 1280x720 and dragon age origins at 1920x1080. COD4 managed a minimum of 39FPS in the ship level with max details and no AA. This game is I believe from 2007. Dragon age, a newer games, suffered, but still remained playable at 1920x1080 with minimum details. I have an athlon IIx4 at 3.2GHz and for the test I limited it to one cpu core. My video card is a radeon 4850.
So, the sempron is not a gaming CPU, but it will work if you are that strapped for cash.
if nt will adding a graphics card like the 9500gt do
If you are currently using integrated graphics that came with the 760 chipset, that is a major limiting factor and no amount of CPU upgrade will increase your framerates.. You want a dedicated video card or you will never get playable framerates. What CPU do you currently have along with your integrated graphics and how much money do you have to spend in total? Perhaps I could suggest something.