It really depends on your usage. Many PS3 games have a forced "install" where sometimes multiple gigabytes of data are copied to HD storage for fast in-game reads (looking at you Metal Gear Solid 4). As well, some games on PSN can be quite large, not to mention that PSN has a few full-fledged retail PS3 games available for download, as well as high def movies.
Having said all that, it looks like Rock Band song downloads are around thirty megabytes a piece, giving you space for ~500 songs on the 120gb model.
Also, the commenter above is correct, the PS3 (including the slim) can be upgraded with any 2.5 inch HD in the future if you run out of space.
Yes, you can backup the drive, assuming you can get it to bootup and run without getting the yellow light. Sony's User Guide on backing up states:.
Some types of backed up data can be restored on another PS3™ system. The types of data that can be restored on another system are as follows:
- Saved data from PlayStation®3 format software *1
- Image files under (Photo)
- Music files under (Music) *2
- Video files under (Video) *2
- Bookmarks added under (Internet Browser)
- Terms that were added using (Settings) > (System Settings) > [Add/Edit Term], or predictive terms that were "learned" by the on-screen keyboard
*1 Copy-protected data cannot be restored. Also, saved data that has been restored may not be usable in some games.
*2 Does not include copyright-protected data.
Most everything should backup and restore, except copy-protectd game saves, movies, and music.
Your best bet for getting your data to the new system is to transfer the data directly from the old to the new PS3 using an ethernet cable. Again, details in the PS3 User Guide. Unfortunately, both systems have to be functional for this to work. After the transfer, any old backups will no longer work. From the user guide:
If you back up data on a PS3™ system and then transfer data to another PS3™ system using the data transfer utility, copy-prohibited saved data that is included in the backup data cannot be restored on either system.
Unfortunately, you can't just swap hard drives between systems. According to this thread requesting this feature, it looks like the PS3 encrypts the internal drive with a key that is unique to each physical system. This key can't be transferred.
Good luck!
Best Answer
IF you mean an external drive or somehow hooking your PS3 to your PC no, you can't install games on an external drive like that. You can however use a laptop 2.5" HDD and replace the physical HDD inside the PS3 (you can backup and restore your current saves to keep all your stuff too).
See this question for a bit more info on the HDD type you need Internal PS3 HDD Replacement