What graphic card i should buy?

Well the GTX 750Ti is the only one that would run on your system, but not that version since your PSU probably doesn't have PCIe power connectors. This one can run on the power supplied through the PCIe slot so it would suit your system better: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127783&cm_re=msi_gtx_750ti-_-14-127-783-_-Product

If you just want the best, get the R9 280 DualX and a new power supply. I would recommend something from XFX, Seasonic, Antec or Corsair (TX, HX and AX only).

The next upgrade would be getting 8GB of RAM and after that a new motherboard/CPU.