How much FPS would I get if I have this Build

You fail to mention screen resolution which makes a massive difference. At 1080p I'd say about 34 FPS average. The HD 7850 is not exactly the best GPU, but it can be overclocked towards 1GHz, which makes a notable difference. Unless you are willing to put money into a better GPU, I suggest you do not play on 'ultra'.

Avaruz is pretty much spot on. Start at Ultra, turn a couple of settings down a little, and you'll be absolutely fine at 1080P. If you want to guarantee solid fps even when things get really busy, you might need to drop a few more settings. It's still a great card.

Here's a very similar build I already had saved, obviously it has a nicer motherboard and cooler for overclocking and bigger HDD, but it's definitely in the same area as your build which is presumably going to cost $550-600 when you allow for PSU and case.
PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD FX-6300 3.5GHz 6-Core Processor ($133.79 @ Amazon)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($32.97 @ Outlet PC)
Motherboard: ASRock 970 Extreme3 ATX AM3+ Motherboard ($84.99 @ Amazon)
Memory: Kingston Black Series 8GB (2 x 4GB) DDR3-1600 Memory ($42.99 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($74.95 @ Amazon)
Video Card: XFX Radeon HD 7850 2GB Video Card ($159.99 @ Newegg)
Case: Xigmatek ASGARD PRO (Black) ATX Mid Tower Case ($44.99 @ Amazon)
Power Supply: XFX ProSeries 450W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($49.99 @ Newegg)
Total: $624.66
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-03-02 07:11 EST-0500)
That's a great entry level PSU but if you want to save some money, a Corsair CX430 is going on Newegg for $25 after rebate http://www.newegg.com/Product/Product.aspx?Item=N82E168... It's hard to argue with that kind of value.