DDR 3 RAM - as much and as fast as I can afford to buy. So what brands/speeds(MHz?) should I be looking at? Also I have heard a lot of talk about 3 lots of ram (like 6GB rather than 8GB, or 12GB rather that 16GB) for PCs. How and why would this make sense assuming a 16bit OS. Would this mean leaving one RAM slot empty when there are 4, or only having 3 slots to start with.
Yup, lots of RAM. Kingston, Corsair, OCZ are all good brands. The speed of the RAM won't matter that much - DDR1333 or better.
You want to get a mainboard that supports the triple channel architecture so it will have either 3 or 6 slots. The triple channel architecture is why you want 6 or 12 etc and it does improve performance. I'd get at least 12GB - more if you can afford it / get a mainboard to support it.
I don't know what you are getting at with "16bit OS" - you will have a 64 bit OS.
Hard Drive - As fast as I can afford (my personal backup stratergy does not involve masses of internal HD space) I am thinking a Velociraptor as they seem popular for the task. Apart from SSD'd is there anything faster/better than Velociraptors? Also, what is the purpose of a scratch disk and do I need one? How much difference will it make and what does it actually do?
More RAM means less need for a scratch disk, but you will still want one.
If you're not going with SSD (and that's a whole other debate), then a VRaptor is a good choice. You could consider putting it in RAID for extra speed/redundancy etc, but unless you're technically minded I would suggest not bothering. Ensure your data is an a different drive to your OS (not jsut a different partition) and yes I'd get a scratch disk (again, another disk, not just a partition) and make it a VRaptor, too (it can be smaller, though - 74GB for example is fine).
CPU - I am thinking i7, but salespeople seem to try and either sell me something else, or one of the older i7's. I know the top of the range i7's are very expensive so I would avoid them. But what would be the pick of the current i7 range in terms of value vs price? How do you pick the best motherboard for your needs/CPU - what do I look for??
Unless you want to spend a lot more and get into a whole other class of performance, then an i7 is an excellent choice. Usually the second from the top is the best price/performance processor. Mainboards? Look for triple channel memory support, look for intel chipset, look at brands like ASUS, GigaByte, Intel itself. The mainboard is important - scimping there can lower performance and reliability.
Graphics card - So how much difference does it make? I have not settled on a RAW processor for my 7D files yet, so I can't say which RAW software I will be running. Possibly C1pro or LR. Obviously I will obviously be running photoshop (will buy CS5 when I get the machine. I have no interest in video editing, watching or gaming - just processing photos as quickly and easily as possible. Any suggestions as to the best brand for my purposes. I have heard ATI are better for image editing and Nvidia for Gaming. What is the differences to look for in one 1GB card over another apart from price. They all look the same to me. I woulod like a better understanding before I choose.
I would get an nVidia - I've always preferred their driver and their multi-screen software leaves ATI for dead. In terms of PS performance, any of the newer 1GB cards would do you well whether it's from ATI or nVidia. I wouldn't be spending more than about $200- and I'd get one with a fan (fanless are nice and quiet, but heat is an issue).
You'll likely get plenty of replies and people will provide specific examples of what they have experienced as being good and bad - it'll be worth reading as there are some really expert folks around here. I hope the above gives you some general guidance to apply to any specific recommendations.
BTW, if you're going to get CS5 anyway, I'd suggest seeing how you like ACR for raw conversion and workflow before spending money on anything else.