Legal or not (they actually occupy a grey area), mod chips will definitely void your warranty.
For my Dreamcast, I own a CD that allows me to play any region Dreamcast game on it -- the CD loads and acts like a patched version of the Dreamcast firmware, allowing any game to start. Unfortunately, finding things like this for cartridge-based consoles is usually a lot harder. SL's suggestion is, alas, probably the easiest solution since Japanese <-> US voltage converters aren't hard to find (my 115->100V step-down cost me $19 at Fry's), and Japanese consoles are also NTSC, so they should work with most composite monitors and television sets.
I have a number of Japanese home computers that work just fine here with my USA setup; all I do is convert the voltage (even this probably isn't necessary, but definitely recommended as extra insurance to prevent early demise of the components, especially if it cost a lot to import -- mine did
).
"you're a doctor.... and 27 years.... so...doctor + 27 years = HATORI SOHMA" - RoyalWing, when I was 27
"Al hail the forum editting Shooby! His vibes are law!" - Osaka-chan
I could still be champ, but I'd feel bad taking it away from one of the younger guys. - George Foreman