Hello Carter...
IMHP, you should stick with 32bit system for now, even if you choose to go with a dual-processor. The main issue with choosing a 64-bit OS is that then you would want to buy 64-bit Apps to run on it. And you might have problems finding a 64-bit version of the apps you want.
About XP, you can run a 32-bit version of Windows XP PRO on a dual-64 bit processor, like the AMD Turion64x2 processor, found in some Gateway laptops. I have been running Windows XP Pro on my Gateway AMD Turion64x2 laptop since the spring of this year. Aside from one hard drive failure, it has been a stable system. But the tech that set it up did say that the hardest part was locating all the correct drivers needed by all the specific components, so he advised me to make a DVD backup of the drivers folder, just in case we had to do this again.
If you select Vista, I would recemmend that you get the 32 bit version of Windows Vista Business Edition. I suspect that that version is more robust and fail safe than say, the Vista Home Edition.
You *can* run 32bit Windows Vista Business Edition with just 1GB RAM, as I'm doing that right now on a 2nd laptop, which is a Lenovo IBM Thinkpad. But, I dont recemmend that. On my Thinkpad, out of the box, the system was starting up over 70 processes. So, just booting the laptop, and getting logged in, but before running any apps, my system was already using up about 70% of that 1GB of RAM. Which doesnt leave a lot of space for major apps like Photoshop & your (big) digital image files. Eventually, I had to go in there, to Control Panel & Admin Tools, and (carefully) turn off quite a few non-essential services, just to get the # of background processes down to about 60. I eventually plan to upgrade to 2GB of RAM on that laptop.
So, my suggestion is to let 2GB of RAM be your MINIMUM, whether you choose XP Pro or Windows Vista. 3 or 4 GB RAM would be better (of course), but, in my opinion, 8GB is probably overkill.
I would also recemmend that you set the Page File on the system to be 2.5 X your amount of RAM. That way, if the kernel does detect that you might be getting close to running out of space in RAM, it can begin moving idle unused code from RAM to the Page File, and it will have plenty of page-file space to work with. That might be important if you start working with multiple # of image files with a major app like Photoshop, all in the same session.
If you dont have enough RAM, you'll see a lot of what's called 'disk-thrashing', as the system continually moves data back and forth from RAM to DISK (page file), and back again. That condition REALLY slows down the apps you are trying to run, and performance will greatly deteriorate.
Another issue with Vista is that some older pieces of equipment, like older HP flatbed scanners, are not supported under Vista, but are supported under XP Pro. (I found that out when I bought a used HP Scanner from a local thrift store. I found drivers for it for XP Pro, but HP said it would not support this product under Vista). This might be an issue for you, if you have older peripherals, like that scanner. In that situation, XP PRO would have the advantage.
Another suggestion is to *not* use disk RAID, as that, IMHO, is simply overkill. Instead, use that disk space for all your photography projects. It sounds like you are a pro, and will therefore be working lots of shooting projects. Its going to take a lot of space to store all the pics, associated files, and secondary files for each project. If you dont use RAID, you'll have 2X the practical disk storage capacity.
Finally, make sure you get at least a DVD burner drive, so you can make regular system backups of all your photography project folders. And a package like NERO 8 will provide all the tools necessary to back up your pictures on a regular basis. If you are going to be using this as a commercial system, it is wise to make a full backup
ONCE A WEEK. You can use USB Thumb Drives to make short-term temporary backups of the projects you actually work on each day. Then, on Saturday or Sunday, run your weekly backup to DVD Disk, and backup everything.
My experince has been that both XP PRO and VISTA BUSINESS EDITION are stable operating systems. The VISTA has a lot more Flash & Polish, and some extra (sometimes irritating) security features. But you pay for the extra functions with more background processes that eat up more memory and compete for your system resources.
Probably the most irrating thing I've seen yet in Windows Vista is what I call the account level audit control. Unless you turn it off (and you can), Vista prompts you to confirm (ie, give your permission) for it to do what you just told it to do.
For example, if you run Spybot SD (anti-spyware freeware), and you run the program from the deskstop, Vista stops and prompts you to Confirm that you really want to run SpyBot. Likewise, if you do a simple task like create a new folder & rename it, Vista runs you through a gauntlet of about 4 additional prompts, each asking you 'are you sure?'. And you have to answer affirmatively to each query, BEFORE Vista will create that new folder and rename. This is a security feature, so no hacker can connect to your system and run something w/o your "OK". But it sure gets tedious after a while. In fact, I will probably turn that off in the near future.
So, to summarize my suggestions...
1. Use a 32 bit OS even if you run 64bit single or dual processor.
2. Consider 2GB of RAM the Mimimum. 3 or 4 is better.
3. Dont use disk RAID. Use secondary disk drives for project work.
4. Get DVD Burner, and do regular weekly Full Backups.
5. If you select XP, get XP PRO w/SP2.
6. If you select Vista, get Vista Business Edition.
7. Check frequently for new updates for your Windows Updates, your anti-virus software updates, and your anti-spyware updates.