If someone has offered you such a trade then many would consider it good. The N64 has gained something of a nostalgia factor in recent years for reasons I can not fathom; personally I think the N64 has aged horribly and any game you want to play on it you would be better off finding remade versions of (the zeldas all got remade, most of the Rare (as in dev) games got ports and enhanced remakes to some flavour of xbox, perfect dark beats goldeneye for me as is on XBLA, mario kart tracks got ported to a lot of things and so on).
That said if nothing else you could capitalise on the trade -- get it, flog the games and N64 on and then buy another DSi xl and cheapo flash cart before pocketing the difference if you miss the DSi.