Unless you are copying files back and forth between drives, doing high data transfers like video or running a heavily used server, you probably don't need scsi. You can get many scsi devices such as a cdr, dvd, hd, scanners, and alot of other devices but if you only plan on buying a scsi hd, it might not be alot of use.
The cost/benefit principle fits here. You can get a much larger drive in IDE than SCSI for less money. SCSI is faster but in apps it only really comes into play in loading of games, copying of high density files, data transfer from digital video, and other similar apps.
What is your primary use going to be? If you presently have scsi hardware, then go scsi or if you think you will do alot of scsi in the future, then go now cause you can always use the hardware again. If not, then get an ide card cause they're cheap and allow you to add more devices now that are more affordable than scsi. Scsi will not make your backups much more secure than ide will cause I've seen scsi 10K drives go belly up. If I had a choice, I would go scsi but I'd rather have some bucks in my wallet more than that. I'm actually thinking about going scsi for everything except hard drives cause they are still too expensive for my tastes.
Whatever your choice, make sure you keep in mind what you will be using your devices for. If it's for gaming, then scsi probably isn't a good choice much more than a better cpu, ram, or a vid card would do you. If it's for professional work where speed is a factor, then go scsi.