I am running a flat-panel iMac 17" G5 2 GHz PPC with OSX 10.4.11 with 2 gig RAM. The machine was new 4 years ago when HDV was first coming to consumer camcorders. I learned the stuff that follows the hard way.
I will try to take on each of your items in order.
You need to increase the RAM. 512 meg is OK for running the operating system and was probably the stock RAM that was included with the machine in the box, and maybe a few lightweight applications - and a browser. Editing video - especially high definition video - is very demanding on the CPU - and LOTS or RAM and available hard drive space is required. You need to increase the amount of physical RAM you have. It should be at a minimum of 2 gig. This will mean, most likely, replacing the 2-256 meg DIMMs with 2- 1 gig DIMMs. If this is the Mac I *think* it is, it maxes out a 2 gig RAM. A new, current, machine running an Intel CPU will be very helpful - but not a requirement if you have your expectations set appropriately.
iMovie 08 has received many complaints. It is less-featured than its predecessor. Version 6 is available for download for you to use:
http://support.apple.com/downloads/iMovie_HD_6
The reason iMovieHD can - but can't - recognize the HV30 - or any other miniDV tape based camcorder when you try to import HDV is because a codec is missing. iMovieHD will recognize and deal with DV (as you have found) with nothing additional. The codec you need to install is the "Apple Intermediate Codec" (AIC). Using the OSX system disc, do a "Custom Installation" of Quicktime Components. In there, checkmark the "Apple Intermediate Codec" and finish the installation.
Capturing DV is in real time. It is 1/4 the information of HDV. HDV will not be realtime unless you get a faster computer. My iMac, at 512 meg, captured HDV at about 1/8 realtime with regular bursts into 1/4 realtime. After upgrading the RAM to 2 gig, it regularly captures at about 1/2 realtime to occasional dips to 1/4 or even 1/8 realtime. Yes, it takes a while to import HDV, but I generally do not sit there and impatiently stare at it - I design the DVD artwork or the DVD menu background or mow the lawn or clean the pool... If I MUST have real-time import, then I need to use a new, fast computer with a Core 2 Duo Intel chip. One of the new iMacs would be fine. A MacPro tower is overkill, but will be REALLY fast.
Importing one hour of DV will use about 13-14 gig of computer hard drive space. Importing one hour of HDV will use about 44 gig of computer hard drive space. You should NEVER allow a hard drive to fill completely - it is STRONGLY suggested to ALWAYS have at least 20% of hard drive as available space. An external hard drive (minimum 250 gig for DV and minimum 500 gig for HDV) is STRONGLY suggested. This entire paragraph is for Windows and Macintosh computers.
When the camcorder is connected to the computer with the firewire cable - and the AIC is installed... when you launch iMovieHD and name the file and select the hard drive destination for the iMovie Project file, there is a drop-down menu that allows you to select DV and DV widescreen - and after installing the AIC, you will have the added selection of HDV.
On the Windows side, MovieMaker running under XP cannot deal with HDV. Only the MovieMaker version running under Vista (Business or Pro, I think) can. If you want to stay in the XP environment, there are a number of applications that can handle HDV, but the same cautions apply: Your Vaio does not have enough RAM, the processor is on the slow side, use an external drive for the video project storage, it will not be a real-time import process.
I've been trying to think of a useful analogy to what you are trying to do - essentially using older computing platforms to handle very high calculation intensive data processing with a brand new camcorder... The best I can come up with is that you are wanting to very quickly heat up microwave-able food - but you are using a conventional oven... At least in the case of these computers, there are some things you can do to get it to work, but without replacing the computers with new ones, things will take a little longer - even if you have the right tools.