"..If I use a standard mini DV tape in the video camera it will automatically down convert from HD.."
Completely wrong, I'm afraid! ..You've been misinformed!
HDV (high-definition DV) uses some clever compression (called MPEG-2, same as is used on normal DVDs) to squeeze hi-def video down to the same 'data rate' (..speed of flow of data..) which normal DV uses.
This means that HDV can be recorded onto
normal miniDV tapes at the same data rate as normal DV. So when you put a normal miniDV tape into an HDV camcorder, the camera can record hi-def HDV (..with 4x the detail of normal DV..) onto it, and it happily records that hi-def video onto normal DV tapes at exactly the same 'data rate' as normal DV ..but with 4x the detail in it.
So when you put a standard miniDV tape into an HDV camcorder, you can use it for recording either - or both! - normal 'low-def' DV or/and hi-def HDV. You can use the same tape for either, and the tape travels at the same speed through the mechanism, so you still get 60 minutes of recording on one tape whether you record HDV or normal DV. (..There's no Long Play, slower tape speed, when recording HDV ..so you can't get 90 mins recording at LP speed except with
standard DV, not HDV..)
So the kind of tape you use doesn't make any difference: it's just the setting you choose on the camera; either HDV or DV.
"..I thought I read somewhere that the import time is much longer and the files is much bigger.."
On older, slower computers the import time for HDV
can be longer than for normal DV, but not necessarily on newer, faster Macs.
Yours is a "..2 years old Core Duo 3 gigs ram.." so that will probably import HDV in "real time". (..The reason for the longer wait while importing HDV on slower Macs is that the HDV has to be converted to Apple's 'Intermediate Codec' kind of video in order to be able to edit it in iMovie. Remember, HDV is
compressed, so it has to be uncompressed during the import so that all the frames of video are restored to being fully present. On an older, slow Mac, this conversion may make importing up to 4x as long as importing normal DV. On a modern fast Intel Mac it should happen fast enough not to notice any difference..)
And yes; file sizes once the material's been imported
are larger for HDV, because there's about 4x as much data there, when it's been uncompressed!
"..If it down converts, will I be able to use the standard 4.7 gig dvd discs when creating the the DVD. ."
iDVD presently 'downconverts' the high-def video when you bring it into iDVD for burning a DVD. (..But iDVD then compresses everything again into the same compressed MPEG-2 format, so that you don't have to think about
file size when wanting to squeeze your video onto a 4.7GB disc; all you need to think about is
running time, as iDVD can squeeze up to 120 minutes onto a normal 4.7GB DVD..)
So if you want to burn HDV with iDVD on your Mac, iDVD will knock everything down to standard def before burning, as Macs don't (..presently..) have Blu-Ray burners built in (..nor HD-DVD burners..) for burning hi-def discs. (..Though it is possible to burn an HD-DVD with a Mac's normal 'SuperDrive' burner by using 'Toast' or Apple's 'Compressor' and 'DVD Studio Pro' ..those discs don't necessarily play on any non-Mac player, such as an HD-DVD or Blu-Ray player..)
However, it's possible to use 'Toast' (..a separate program..) and an external Blu-Ray burner to burn hi-def videos, created with iMovie, onto blank DVDs which
will play in a normal Blu-Ray player. (HD-DVD players have been abandoned by the manufacturers - mainly Toshiba - as Blu-Ray has become the accepted standard for hi-def DVDs.)
So, in summary;
1 - It makes no difference which tapes you use: the tapes don't switch the format between DV and HDV. You switch the format by choosing a setting
in the camera; the tapes are immaterial.
2 - Your fairly modern Mac should import HDV in pretty much "real time" with no delay, I should think (..though you don't say exactly which model you have). However, you're right that HDV video, once imported, takes up more space than standard-def 'normal' DV, so it might be wise to buy a huge-capacity FireWire-connected external hard disc to store your hi-def projects on. (When you buy one it'll probably be formatted to use with a PC; you'll probably need to reformat it as 'Mac OS Extended' to make it work with iMovie. If it's a really large capacity disc, like 500GB or 1 terabyte (1000GB) you may also need to reset it as a single 'partition' as part of the reformatting, but you can find out all about that by doing a search in OS X Discussions, or looking at Mac Support pages.)
3 - iDVD presently down-converts all hi-def to standard-def. If you want to create hi-def DVDs you'll probably need to buy '
Toast' (click on that link) and get an external Blu-Ray burner. That'll need a fast connection to your Mac, preferably S-ATA.
