First, the D800 uses parallel ATA (PATA) IDE drives, so you cannot use a SATA drive of any type or model.  Second, the D800 BIOS never received an update to 48 bit LBA so any drive larger than 137 GB will not have BIOS support.  The largest drive you can easily use is 120 GB IDE.

If your operating system installation CD supports 48 bit LBA, such as Windows XP Service Pack 1 or later, you can still use a drive larger than 137 GB.  Windows XP SP1 and later use their own disk access routines once the boot process has finished so having BIOS support is not as critical as it once was.  Since the boot process does use the BIOS for initial access, however, it is possible to have a disk crash that can force you to rebuild the boot drive.  Also, any utilities that use the BIOS routines can cause problems as they will not be able to see the entire drive.  If you wish to use a drive larger than 137 GB it would be a good idea to keep frequent back-ups.  You may also alleviate the pain of a crash to some degree if you partition the hard drive into at least two partitions, keeping the boot partition within the 137 GB boundary.  You can then use the second partition for data storage which will be left untouched by any disaster that occurs within the boot partition.

What about the various adapter cables and software that I see offeredthat claim to solve the IDE to SATA conversion?  Do they work?

There are some that work with a desktop, but I don't see how you would get an adapter for the internal drive in a laptop.  You can use an external drive shell if you wish that attaches to the computer via a USB port; the type of drive you would use would then depend on what type of drive shell you purchase.  I would not expect a USB interface to be terribly fast, however.

There is simply no way to use a SATA drive inside this system.