Is there a way to tell if a program is filmed in SD to conserve space?

There's an "HD" flag that shows in the guide, but it's not entirely reliable (some HD shows don't have it, and a few SD shows do). You can specify it in Wishlists by choosing Category > HD. To make it a preference, create two WLs, one with the flag and one without, with the flagged one having higher priority.

I used to do all that, but now I just take the SD channels out of the guide. Most shows are in HD now anyway.

With a premiere XL you'll be hard pressed to fill it up, even with HD, so I wouldn't worry about it. 1TB = ~140 hours of HD programming, that's almost 6 days of TV if you watched non-stop. So unless you plan to keep entire seasons of shows space will never really be an issue. Plus if you really are pressed for space you can always pick up one of those DVR Expander drives for ~$120 and double your space to 280 hours of programming.

Dan