With a $130 budget, you are looking at something like a 6670, a nice jump in graphics power.
1) Will it fit in your case? Check the dimensions of the card vs. the space in your case. If your pc has a small form factor your gpu choices will be more limited.
2) You presumably have a pci-e x16 slot.
3) Go to the amd web site to get the requirements for your card. The 6670, for example needs a 450w psu with one 6 pin power lead.
Your psu will have a ul required data plate that enumerates the specs.
Because you are using a ati card now, I suggest you restrict your pick to another ati unit, not nvidia. That way, you will not have to change drivers, and can just drop the new card in.