Let's start from the beginning.
1: Is there any benefit to having a dedicated swap partition?
2: What size partitions should I use?
A: Whatever you need for the programs you want to install, and probably a little extra. How much extra depends on how much you think you'll need. Personally, I'd allot more space to XP than DOS, since DOS programs were generally very small, whereas Windows programs are ballooning in size every day.
3: Do all partitions need to be FAT?
A: No. In fact, DOS can only handle FAT12 and FAT16, and since FAT16 has a size limit of around 2GB, it won't be very useful. XP should be NTFS, and you may have to partition your external drive, creating a special 2GB FAT16 partition just for DOS.
As a bit of an aside, I don't think installing DOS is going to be such a good idea. Like I said, it can only read FAT12 (primarily used for floppies) and FAT16. This would mean you wouldn't be able to read anything from the XP partitions, unless you made a large number of 2GB FAT16 partitions.
You would probably be better off getting a Knoppix Linux CD, or some other bootable CD Linux distribution. These can be great help in dealing with a damaged operating system. The entire thing runs from the CD-ROM, something your system is all but guaranteed to be able to do. I had a 486 that could boot from the CD-ROM drive, just like every other system I've owned/built, so yours should be able to as well. At the very least, a bootable Linux CD should get you read access to your NTFS partitions, and then let you copy the files to your external, provided it's formatted as FAT32. I'm not sure on the status of NTFS write support in Linux, and more specifically, the latest Knoppix version, so it's probably best not to count on it.
Knoppix CD images are a free download from their website, and I think they sell commercially pressed discs that will last quite a bit longer than a burned CD. You could just be sure to keep this disc in a safe place until such time that it's needed.
Alternately, if you insist on going the dual boot route, you could replace DOS with Linux in your plan. You could do a minimal install, so it doesn't need a lot of disk space, and have it ever at the ready to copy files off of your Windows partition to your external. The major benefit to this method over a bootable CD, is that if your system has a CD/DVD burner, you could use that as another method of backing up files.
I'm wiping my laptop HD and starting fresh. I want to install DOS and XP, and I've gotten a lot of tips through searching old posts, but I still have a few questions. There wasn't much out there for this particular combination.
First, equipment. I've got a Toshiba Satellite M55 with a 75 GB HD. I'm installing HP Home. I have no floppy drive. I'm also using a 250 GB USB external hard drive for backups and such.
Partitions: What size for each? I was thinking I'd have one for DOS, one for XP and applications, and one for data, although I'm planning to store most of my non-essential data on the external. Is it beneficial to also have a separate partition for the swap file? I read a recommendation in another thread for a 30 GB volume for XP/apps and a 2 GB volume for the swap file with the rest allocated for data, but I didn't find any recommendations for MS-DOS volume size.
FAT / NTFS: I know that I'll need the DOS volume to be FAT and that I'll need to install DOS first. Will the other volumes need to be FAT too, in order for anything there to be accessed by DOS? I think I'm gonna need to save video files >4GB, so that wouldn't work too well. Unless I could have a small partition formatted NTFS just for video files? Once downloaded, most of the files would probably be transferred to the external, but they'll need a temporary home on the internal. Also, it would be ok to format the internal FAT and keep the external NTFS, right? They can work together?
I don't really plan to use DOS much, mostly for emergencies. For instance [back story], next time Windows crashes and will only boot so far as the splash screen before giving me the BSOD, I'd like to have a way to operate this beast. I'd also like to be able to run programs in DOS without Windows unnecessarily hogging all the resources. I miss the option of starting in MS-DOS mode; this would all be unnecessary. I repaired the install to get myself back to partial functionality - just barely enough to copy my files - which in itself was a mess. I'm no longer able to COPY or PASTE or drag-and-drop, so I had to go into the MS-DOS prompt and XCOPY everything. From this point forward, obsessive backups.
Also, I'd like to point out that I'm the worst kind of computer user: the kind that always thinks they know what they're doing, and thus tends to screw things up much more than other people. I've done tech support; I know and hate people like me. For that, I apologize, and also if this is too long and over-explained.
So, how shall I proceed?