ohhhh…you guys are making it difficult…did I tell you this weekend here the large retail chain is offering 10% stock including Macbooks (which apparently is not frequent)
ok - so can I ask re the Macbook option:
1 - How much HDD does all the normal web development s/w + Parallels take up? That is so I can work out how much of a HDD upgrade I should go for if I go the Mac? (e.g. Ruby, RadRails, Dreamweaver, Photoshop, Parallels)
well, stock is 80gb but you can get up to 200gb. I really don’t know about the space constraints you are requesting. I have a lot hdd space that I never keep track. There are many factors for considering the appropriate space and needs.
2 - Are there any applications that come with the Mac that you’d normally have to pay more $$ for on the PC platform that are worth it?
i.e. do I get more effective value for money with the Mac here? (in particular from a developer point of view)
Uhm, you might want to take a look at the OSX downloads section on apple.com and MacPorts, a ton of opensource software for free. From a developer point of view, well… what do you really need these days but Eclipse? Or TextMate. And if your into java or objc development you get Xcode free, but I don’t think that it is installed by default. My main dev tools are TextMate for python and ruby, and vim for just about anything else. Again Free!!! TextMate is not free but well worth the cost.
3 - As an aside as a muso, does GarageBand do Audio & Midi recording? Or would I need to spend more money to get a Mac version of something like Cubase / Protools?
Yes. You will be amazed with GarageBand, I only wish that when I was younger and though I would be a rockstart I had such an awesome recording tool. Okay, so I still think I am a rockstar and plug my guitar right into my mac and lay down track with ease. Not to mention all the JamTracks GB has.
tks heaps - still have the background thought of my backpack being heavier 
Sounds to me like you actually have no idea about OSX. Let me end with this. Once you use OSX coming from windows… say maybe a 2 week learning curve, after a few months of use, you will be asking yourself, Why have I not done this sooner, and Wow, I knew Microsoft was not the best OS, but I really didn’t know it was that bad.
I make those as promises to you. Feel free to let me know after a few months, if I was correct.