I'm looking at an ATI Radeon HD 6850,
I've a feeling my motherboard, a Asus P8H61-M LE/USB3 Basic Micro ATX Motherboard, will be limiting me a bit here t
Your motherboard is not holding you back if the HD 6850 is the max your budget will allow you to spend for a graphics card.
What is the resolution of your monitor?
What do you have for a PSU?