The Antec 900 is a standard ATX case. Any ATX power supply will fit. Meaning 99% of the units on the market.
$125 after rebate. XFX 80 Plus Gold and fully modular. This unit like all XFX units is made by Seasonic. It's based on the Seasonic X series arguably the best power supply series on the market and saves you about $40 by not buying the Seasonic brand.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817207028&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
You sure you need 850 watts? Check here.
http://www.realhardtechx.com/index_archivos/Page362.htm