Unless the second hard drive has a DOS logical volume in some sort of DOS partition in addition to the Linux partition(s) neither XP not DOS will have any idea that it is there and no way to access it. Your drive letters won't change. FDISK will see it as a "Non-DOS partition" (remember that FDISK sees your first hard drive as Drive 0 and your second HDD as drive 1).
If you DO choose to setup a DOS logical volume in some sort of DOS partition on the second HDD (I see no reason to), then your present setup will see it as D: and the CD-RW and DVD will each move up a letter.
Linux will have no difficulty in seeing DOS logical volumes in your DOS partitions, whether FAT-16 or FAT-32 and you can set Linux up to automount directories of interest on boot.
DOS (and Windows, including XP) assigns drive letters by first looking for primary DOS partitions and assigning them driveletters; then, and only then, will it look for logical DOS volumes in extended DOS partitions and give them driveletters. Once all of the hard disk partitions have been assigned drive letters, the system will allocate letters for other devices that are operated using drivers like CD-ROMs or Zip drives.
[Note that in Win98SE (and DOS, using MSCDEX.EXE invocation parameters) you CAN change the assigned letter of driver-accessed drives like CD-ROMS and in XP you can change almost all of them (be careful you don't confuse yourself, the OS and installed programs). Note also that emergency boots from floppy or CD may not give the drive letters you have gotten used to expecting and be extra careful checking in this situation.
So suppose you have one physical hard drive in the system, with a primary DOS partition and a logical DOS volume in an extended DOS partition - DOS will assign these as C: and D: with a CD-ROM as E:
Add a second hard drive, similarly partitioned and DOS will allocate drive letters as follows:
C: 1ary DOS partition on 1st physical HDD (you can't subdivide 1ary partitions)
D: 1ary DOS partition on 2nd physical HDD
E: 1st Logical DOS volume in extended DOS partition on first HDD
F: 1st Logical DOS volume in extended DOS partition on second HDD
G: CDROM
This can be annoying and confusing, particularly if one is using a removable HDD. The solution is to create only an extended partition on the second or removable HDD (DOS doesn't HAVE to have a primary DOS partition unless you intend to boot from the drive). You must create at least one logical DOS drive in the extended partition (actually you can create as many logical DOS drives as you can spare driveletters and have space for if you want). So:
C: 1ary DOS partition on first HDD
D: 1st Logical DOS volume in extended DOS partition on first HDD
E: 1st Logical DOS volume in extended DOS partition on second HDD
F: 2nd Logical DOS volume in extended DOS partition on second HDD
G: CDROM
Of course, all DOS logical volumes (whether on a primary DOS partition or on an extended DOS partition must be formatted before you can use them.
FAT-16 restricts you to 2GB partitions, FAT-32 allows sizes up to 2 terabytes.
With SCSI drives Windows assigns fixed-disk drive letters like so:
1st -> Primary partition of Primary master
Next -> Primary partition of Primary slave
Next -> Primary partition of Secondary master
Next -> Primary partition of Secondary slave
Next -> Primary partition on SCSI ID 0
Next -> Primary partition on SCSI ID 1 (and so on)
Next -> All Logical DOS drives of partitions on Primary master
Next -> All Logical DOS drives of partitions on Primary slave
Next -> All Logical DOS drives of partitions on Secondary master
Next -> All Logical DOS drives of partitions on Secondary slave
Next -> All Logical DOS drives of partitions on SCSI ID 0
Next -> All Logical DOS drives of partitions on SCSI ID 1 (and so on)
Next -> CD-ROMs, Zip drives etc accessed through device drivers, letter depends on order in which device drivers are loaded)
So ALL Primary partitions receive drive letters before ANY Logical DOS drives do.
Most people don't bother to have anything but a big primary DOS partition on their Windows systems because they get confused or don't know how to do it. It's actually quite simple and very useful if organised sensibly and not overdone.
Unfortunately, by the time people realise that partitions would be useful and wish they had 'em they've been up and running for a while and don't want to FDISK/reformat/reinstall everything or cough up $60 for a copy of "Partition Magic" that allows you to repartition drives non-destructively and more or less on the fly.
Hope this helps (and open to additions/corrections).