You have two problems:
First, you need a low profile card.
Second, you have only a 220 watt PSU.
A 6850 needs 9 - 10 amps just for itself.
Looks like the most power AMD low profile card is a 6670. Good news is that a 6670 only needs about 65 - 70 watts so your present power supply should be able to power it, but you will have very little power in reserve.
Is the 6670 good for gaming?