If you want to stay 100% legal then you can't use a rom at all. Not one ripped yourself, or one downloaded.
Her is a site with a lot of information if you feel like reading about it:
http://www.worldofspectrum.org/EmuFAQ2000/AppendixB.htm
Here is a quick example:
Atari, Inc. v. JS&A Group, Inc. 597 F.Supp. 5 (N.D. IL, 1983)
JS&A Group were the vendors of a cart dumper called the "PROM Blaster" for Atari 2600 videogames. It was a device designed to allow its operators to dump the object code from within the ROMs of an Atari 2600 videogame cartridge. Atari sued JS&A for contributory copyright infringement of its proprietary videogame software. JS&A argued that archival copying was permitted under copyright law (17 USC 117), so the purpose of a cart dumper constituted "substantial non-infringing use."
The court found that a computer program embedded within a piece of hardware, such as the ROMs used within a typical videogame cartridge, cannot be reprogrammed or erased. As such, it was contained within a form of storage media designed to permanently preserve the program. JS&A's contention that cart dumps protected the actual cartridge against possible physical harm (and thus the program embedded inside) might also be applied to other forms of physical media, such as phonograph records and books. Since copyright law did not allow for this practice in regard to these and other such forms of physical media (photocopying a book, physically duplicating a phonograph record), they likewise did not apply to the practice of archiving computer programs embedded within a piece of hardware. The archival exception for computer software did not apply to programs stored within permanent storage media (in this case a videogame cartridge) because these forms of media are not subject to the sort of risks that the archival clause was designed to guard against.
As a result, the court ruled that dumping a videogame cartridge for archival purposes is not covered by the archival clause of copyright law.
Alright quick is relative but anyway, from what I understand there is no legal way to backup your games and everything contrary to this is just rumor and people trying to justify their copying.
And like rooker said get a action replay, any way you use copyrighted roms is illegal. That is if you want to be 100% legal.
Take care,
-Archi