|Based on a quick RS to USD conversion, that's about a $40 budget. With that you really won't get very far. |
I would suggest getting a Radeon 6770 or something alike it, it sounds like it would fit what you want. They are about $80, so double your original budget.
No, the cards won't help each other out. If you have your monitor hooked up to the old card's ports then it will only use the old card, and likewise with the new one. If you had two AMD graphcis chips they have something called CrossFireX, which allows the memory/GPUs to be combined. I'm not sure if Nvidia offers something like this, but if they do, I would suggest getting another Nvidia graphics card. Because the one I suggested and the one you have are different brands, they will never be able to work together.