Sounds as though you're planning to do what I just did...
<<<Now that the iMac (20-inch) and PB (15inch) have been updated I am going to go ahead with the switch, got quite a few questions, if anyone can answer any I would really appreciate it.>>>
In re-reading this, I realize that you may actually be planning to only go with one of these, not both? Hard to tell, the way this was written. I bought the 20" iMac (rev B) a week ago, promptly fell totally in love with the whole Mac world and its superlative X OS and decided that I just had to have a PB too, for those times when I am away from home and my iMac. I'm typing on the iMac right now, the 15" PB is on its way to me....
1. I am planning to upgrade the ram on the PB to 1GB, what is the difference between the 2DIMMs (2x 512) or the 1DIMM, apart from the price? - I have no idea what the DIMM's are!
Upgrading the RAM on ANY computer you buy is a very wise decision. I maxed out the RAM on my iMac (2 GB) and did the same with the BTO PB coming to me. Basically the difference between 2 x 512 DIMM chips and one 1 GB chip is that you can stick the 1 GB into one of the available RAM slots and have the other slot using either a 512 chip or another 1 GB chip. Someone can buy a new computer which has a 512 MB chip in one slot and then they can choose to add another 512 MB in the other slot, resulting in a total of 1 GB RAM, or they can pop in a 1 GB into the empty slot, then coming up with 1.5 GB RAM, or they can pull out the 512 MB from the existing slot and put a 1 GB in its place and, voila, the machine has 2 GB RAM. At the time i purchased my iMac at the local Apple store, they "bought back' the existing 512 MB RAM chip that was in there because I had specified that I wanted 2 GB RAM, necessitating the installation of two 1 GB chips. Ditto for the 15" PB that I've ordered from Apple online (had to do that because the local store doesn't yet have the new memory chips nor the ability to change the configuration of the machine to use the 7200 rpm hard drive, which I also was requesting.
2. I am going to get MS office for students, do I have to do anything to prove that I am a student?
I was surprised when at the store the sales person told me that I could just go ahead and buy the MS Office for Mac Student/Teacher edition, although I do not fit into either category. No arguments from me, I just paid for the thing and brought it home and installed it.
3. Cases for the PB. I like the look of the Le Bag for the 15" version. Does anyone have any recommendations?
Just ordered myself a Crumpler "Dreadful Embarrassment" bag (yes, it's for the 17" PB but from past experience with Crumpler bags I knew to order in a larger size than would be normally required). I also ordered a Timbuk2 quilted orange laptop sleeve in the medium size. I'm planning to use the Timbuk2 sleeve not only when I'm tucking the PB into the Crumpler bag to carry off somewhere, but also to keep on it while sitting here at home during times I'm not using it. There is a very good thread in the "stickies" area all about bags, sleeves and such in this section of Macrumor forums, so have a look at it!
4. If anyone is familiar with the Egg money card offer in the UK, does the 10% cashback only apply online, and do I have to follow the link given by egg?
In the US here, and am not familiar with the Egg money card offer. Sorry!
5. External speakers for the PB. Will these work ok, ie. Just plug and play? I was thinking of the Harman Kardon Soundsticks II 2.1 speakers, anyone had any experience of these?
My iMac has speakers with it, and so will the PB, so I will just be using those for the time being, but I do think the HK Soundsticks speakers are really lovely and would be a fantastic addition to any Mac system! They do look as though they'd be a plug-and-play setup.
6. I know that the PB comes with more "pro" software than say the ibook does, is there any software that (for example) the ibook has and the PB doesn't, which would be useful to have?
Can't really speak to what software is available on the iBook vs the Powerbook, but I think the items which come as part of the basic OS are going to be the same. At the time I bought my iMac I also bought the Office Suite for Mac Student/Teacher edition, and I bought Photoshop. I'll be loading both those into my new PB when it comes, and they're already in the iMac. At some point in the future I will be adding Studio 8 (Dreamweaver, etc.) and more than likely, Apple's new Aperture. I intend for my PB to serve as my machine-away-from-home with most, if not all, of the functionality and features of the iMac, so that when I am on a trip, for instance, I'll be able to download photographic images and then (if I have time and inclination) work with them right then in the PB, just as I would later on in the iMac, once back home. My ultimate goal is to get a Power Mac, but that's for the future.... in the meantime I am really excited about using my new iMac and 15" PB to fulfill my expectations and needs....
The process of switching from PC to Mac was surprisingly painless and hassle-free. I am still amazed at this. I had really anticipated a learning curve filled with complexity and stress, and that has not at all been the ways things have gone. At the risk of sounding trite, it's very true what they say: everything IS easier with the Mac! It just works!!
Good luck in your decision and in your purchase....
OTB
Thanks loads 
