While knowing about your PSU is essential to determining the safe upper limit, you should be able to install a HD5670 even if it's only 200W. If you are limited to low-profile cards, then a HD5570 is probably the best safe choice. There is a low profile 9800GT, but it may draw too much juice. Tell us what's on the label of your PSU, or its brand and model.
Is there a way to determine the PSU without opening the case? I checked the power cable and the back, but its not saying much.