|I recommend buying a 3.5" with an external power adapter. These are more reliable than the smaller 2.5" self contained.|
All external drives are subject to heat related failure. You need to get one that has good cooling.
As far as what type of computer you connect the external drive to that is not really a factor. If your USB connection is only USB 1.1 as opposed to USB 2.0 the transfer speed will be much slower. This will be more noticable when transfering large files.
One last thing to note. I understand your situation with a somewhat smaller internal drive. However, you should maintain at least TWO copies of any data you wish to keep. All hard drives eventually fail. If you have CD/DVD burning capability I suggest you at least burn a second copy of things like your irreplaceable photos and music you have purchased. Documents you have created, etc. You get the idea?
Do not save any files directly to to root of the external drive. Create folders and save to them. You would be better served to reformat a 320GB drive to NTFS as it is much more effeicent and secure than FAT32.
Many external drives come with preloaded software that will allow you to backup your entire internal hard ddrive, should you wish to. If you backup into a labeled folder you can periodically perform a backup and if you keep two backups you should be fairly safe. After two, you could delete older ones to free up room.
Tho only other thing I can suggest is to buy a brand name drive with a good warranty. Maintain the drive just as you should your internal drive. Periodically running defrag for instance.