your current cpu is socket 1155 and you can only use cpu's that have this socket design
you can get two kinds of Socket 1155 Intel cpu's.
sandy Bridge: those will be socket 1155 cpus with 2xxx series eg, core i5 2400
Ivy Bridges : those will be socket 1155 cpu's with 3xxx Series eg, core i5 3470
for Ivy, you might need a BIOS update. So use your current cpu and perform a BIOS update so that new 3xxx series cpu can be recognised by the motherboard.
newegg page: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=100006676&isNodeId=1&Description=1155&x=24&y=14
you can pick one of those processors according to your budget.