What kind of PSU?
If you have a good 400 watt power supply (28 - 30 amps on the 12 volt rail), the 6850 should be a good choice.
Figure 10 amps (120 watts) for the video card, 10 amps (another 120 watts) for the CPU, and 100 watts for the motherboard, RAM, and a couple of drives.
One of my systems has an OC'd Q9550, 4 GB RAM of DDR2 RAM, a GTX260 - a card pulls about 30 watts more when under load, a Gigabyte EP45-UD3P motherboard, 3 hard drives and an optical, and a Soundblaster card all powered by a Corsair 750TX.
Running 3 instances of Prime95 to load the CPU and 3DMark06 to load the GPU, it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% PSU efficiency, the system pulls 300 watts from the PSU.