Some good advice here.
On the 720p vs 1080i question, my take is that with broadcast, particularly on sports, generally the 720p is more appropriate. 1080i maybe for Discovery, but mostly 720p. Match up with what's being sent. Progressive is generally better than Interlaced. But it does seem like a 1080p, for eventual HD-DVD or Blue-Ray usage, perhaps game consoles, has certain potential long-term advantages. Not broadcast. Make sure to get a set with 1080p input, not all 1080p sets have that.
The LCD vs. plasma debate mostly seems to come down also to what you watch, and how you watch it. Dark room, watching movies, buy Plasma. Lighter room, buy LCD. If you read while watching TV (I know, arcane concept reading ;-), LCD. LCDs tend to be cheaper than Plasma in terms particularly of 1080p sets, up to maybe 42"-46". Plasma is getting cheaper mostly because of LCD, so where LCD is not yet competitive (the 52, 57, 65" LCD sets that are out get into serious $s) price wise, Plasma still itself fairly expensive.
The other thing to watch out for, particularly on LCDs, is that the brands have not really reached the dominance they had before in regular TVs. Definitely watch the HDTV yourself, check out ghosting and angles, see how it "Looks" to you for a variety of sources, sports, movies, and so forth. Is the motion smooth, or does it "cog" or look like a strobed picture. Watch regular DVDs also, as that's probably what you'll be watching, and not all of the TVs scale well (although you could buy a scaling player). Some of the cheaper, off-brand LCDs have some of the more current scaling chips. It's really tricky to tell sometimes. Don't stop looking at Sony and Samsung, make sure to chase down Sharp, Westinghouse, and look at some of the cheaper off-brand stuff.