On a number of occasions I’ve seen, or read about, people dropping SD clips into HD timelines in FCP and just letting the program automatically scale the SD footage up to HD. While quick and convenient it doesn’t generate very good results. If you have FCP you most likely also have Compressor and it will do a much better job of converting the footage than FCP will (especially if you are also mixing interlaced and progressive footage). Going to Compressor is an extra step but it’s well worth it.
The key to getting the best out of Compressor in this type of situation is to use the Frame Controls. By default Frame Controls are turned off so you’ll need to go to the Frame Controls tab in the Inspector Window to turn them on. The quality settings for the Resize Filter and Deinterlace are listed as Fast, Better, and Best. You’ll want to either use Better or Best although be warned that using the Best setting will require wicked long conversion times but the results are about as good as you’ll get without using a hardware-based tool. The Anti-alias and Details Level sliders will require a bit of trail and error each time you use them so I suggest doing some small test clips to dial-in exactly where you want them.