Even though you have an 850 Watt power supply it won't be using 850 Watts all the time.  Here is an article with someone who hooked a PC up to a Kill-a-watt meter to test how many watts are used while the system is running.

http://www.codinghorror.com/blog/archives/000353.html

Personally, I prefer FedEx to UPS.....oh, wait....nevermind.

Q: How are the "sizes" of UPSes determined?

A: Typically, a UPS has a VA rating. The VA rating is the maximum number of Volts * Amps it can deliver. The VA rating is not the same as the power drain (in Watts) of the equipment. (This would be true if the load were only resistive or the circuit were DC, not AC). Computers are notoriously non-resistive. A typical PF (power factor: Watts/VA) for some computers may be as low as 0.6, which means that if you record a drain of 100 Watts, you need a power source with a VA rating of 167. Some literature suggests that 0.7 may be a good conversion factor, but this will depend heavily on the specific equipment. Moreover, there's really no way to determine these numbers besides measuring them.

Note: Some UPSes can continue to deliver power if the VA rating is exceeded, they merely can't provide above their VA rating if the power goes. Some can't provide power above their VA rating at all. Some may do something really nasty if you try. In any case, I strongly recommend not doing this under any circumstances. Generally, the rule of thumb seems to be never drawing more VA from an UPS than about 75% of its rating.

http://www.csgnetwork.com/upssizecalc.html

This is what I use for my system and it handles it without issue ...
http://www.newegg.com/Product/Product.aspx?Item=N82E16842102048

Here is my system data on elements that draw power.
CPU: Intel Core i7 975 Extreme
MB: Asus Rampage II Extreme
RAM: 12GB Corsair DDR3 clocked to 1800
GPU: Dual Nvidia GTX285 SSC in SLI
OS HDD: Dual 10k rpm 74 gb Raptors in Raid 0
Game HDD: 10k rpm VelociRaptor 300GB
Storage HDD: 7.2k rpm WD 1TB
Case: Cosmos S
PSU: 1050W ENERMAX REVOLUTION85+
Combo Drive: LG Blu-Ray / HD-DVD-R/RW

Fans: (11x120mm) 3x Rosewill, 1x A.C. Ryan, 6x Yate Loon, 1x AeroCool Extreme
(6 dedicated to a push pull on the 3x120mm top mounted radiator)
Fan speed & temp controls: Scythe Kaze Master Ace 5.25” 4 channel controller.
Temp monitor, Pump speed, Radiator fan speed controller: Koolance CTR-CD10
Pump: Swiftech MCP655 Variable speed (317 gph capable)
Flow meter: Koolance INS-FM17
Lighting: 3x toggle pannel wired to 6 UV CCFL tubes

I have a similar system (See sig) and it mkaes my APC 750's alarm go off indicating theres no way the UPS could supply it.