I think Gatos' advice is sound, especially since you are working on an edit that isn't "overdone" (like the Star Wars PT, where people now except HD) - meaning, people will be willing to overlook the SD vs HD issue if you produce a good quality edit, and that has nothing to do with picture resolution.
When you are doing your first edit, you really want to make things as easy on yourself as possible. And working with SD will make thins easier. This is especially true when it comes to delivery; and that's incredibly important not only to get your edit approved, but to be watched. You won't be thinking about this now, but HD delivery options on the Mac are much more complicated than SD, since Apple has and is still adamantly against supporting the blu ray format natively. If you are using FCP7, then you also have DVD Studio Pro, which should make it much easier to create your final product (in SD). You can also use iDVD if you wish.
I'm not going to say you can't go with HD, but you really need to make sure what you are doing in terms of format choices, because the last thing you want is to make a bad choice at the start and have to start over at a certain point when it comes back to bite you. Or you get to the end and get overwhelmed by how to author your final product.
If you
are gong to edit in HD, it is
essential that you edit in an appropriate format. Unless you are using a super powerful computer, you should
not be doing your editing with h264 - it's a highly compressed codec meant solely for playback. Your computer has to do tons of decoding work to allow you to work and make editing decisions at the frame level. You really need to stick with prores when editing on the Mac.
Second, if you are using FCP express or 7, i'm almost positive you can't even use h264 for editing - when you import such a file, FCP will first require a transcode back to prores.
FCPX
can edit h264 natively, but again, you need to have a super powerful computer to do so in HD. however, FCPX does allow you to create a proxy version which will be 1/4 resolution and in prores proxy. I've done this in the past and it works fine; you just need to remember to switch back to your native format before final export.
If you set things up correctly, I think editing in HD is not too much harder than SD - as long as your computer can handle it and you are prepared for the authoring/delivery process. But it is
critical not to try and take short cuts to reduce file sizes.
HD is not the be all and end all, and it seems to me that plenty of fan edits are still being created in SD. You also have to remember that SD can be viewed by a much wider audience than HD at this time.
Finally, and I know I said this before, you can always go back and re-release your edit in HD after it has been approved! Lots of people do this, including myself. Yes it sounds like needless extra work, but your firstling edit is almost certainly something you will end up wanting to redo, and if you release it in SD, you get the built in incentive to do so to upgrade it to HD!