Based on your question, I assume you’re using a version of Photoshop prior to CS. Use the photoshop preset for 720×540(601) if your sequence settings in FCP are for 720×486. Use 720×534(DV) if your sequence settings are for 720×480(DV). Set your ruler to percent and drop guides on 5% and 95% for action safe and 10% and 90% for title safe.
For CS or later ONLY, photoshop has built in presets and support for non-square pixels. Just use the one for 720×480 for DV and the 720×486 for other SD 4:3 sequences. Viewing the corrected aspect on your computer monitor while you design is turned on by default. This allows you to draw a circle and it will be a circle on TV, however it also degrades the view a little. This degradation will not truly be in the file, it only shows up on screen in photoshop. If you need to see the full quality, you can turn that off, but circles will be ovals. You can’t have your cake and eat it too.
Also, with CS2, if you have a DV device connected to your computer and to a TV, you can preview your design on tv directly from photoshop. I’m not in front of photoshop right now, but I believe it’s somewhere buried under the “File” menu.
Thanks,
Gary