suggestions for a 500-550 watt PSU?

If you can use the rebates...
- cheapest: $51 XFX 550W: http://www.newegg.com/Product/Product.aspx?Item=N82E16817207038&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
- modular: $60 EVGA 750W: http://pcpartpicker.com/part/evga-power-supply-110b20750vr

If you can't use rebates:
- cheapest: XFX 550W: http://pcpartpicker.com/part/xfx-power-supply-p1550sxxb9
- modular: - EVGA unit above, but get from Amazon instead

On your build, I don't know how tight your budget is or how important gaming is to you, but upgrading your 750ti to a GTX 950 would give you substantially better gaming performance (like 50% faster) for around $30. Worthwhile unless you budget is really tight, or gaming isn't a priority for you.