My only tidbit of advice is the obvious - make sure aspect ratio of the native resolution of the frame matches the aspect ratio of the majority of the intended pictures. All of the moderately priced frames I looked at last holiday season that were not 4:3 did a terrible job of scaling 4:3 source material to whatever strange widescreen format the frame used; however, the 4:3 frames did a better job of letterboxing other aspect ratio images.
IIRC, I ended up getting an 8" 4:3 (800x600) frame from Dell (of all places) for like $160.; my grandmother seems happy with it.
The some of the more expensive frames (with the same rough specs) did have notably better contrast and color gamut (different LCD panels, I assume), so you really need to view some of your own images in person to make a final decision.