Slightly more secure. But the most memorable methods of inserting "random" characters into words are not random at all, and have already been programmed into password-cracking software. o=0 and c=( are already variants used in dictionary attacks. "Every third letter" isn't random at all. Never mind that 7-character passwords are within the brute-force realm where it's possible to try every combination of ascii characters with a current graphics card.
If you want memorable, you're better off going long with nonsense phrases that have no meaning except to yourself and have not appeared in print or on wikipedia. (The "correct horse battery staple" method.) Have your music player spit out four random song titles and pick a word from each "mothra lust mirror coffee," or scan your bookshelves and pick four words from different books "india effect stones goblin." Adding two characters gives you more bang for your buck than *randomly* (with dice) substituting one. Non-random l33t substitutions don't help much at all.
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If you want memorable, you're better off going long with nonsense phrases that have no meaning except to yourself and have not appeared in print or on wikipedia. (The "correct horse battery staple" method.) Have your music player spit out four random song titles and pick a word from each "mothra lust mirror coffee," or scan your bookshelves and pick four words from different books "india effect stones goblin." Adding two characters gives you more bang for your buck than *randomly* (with dice) substituting one. Non-random l33t substitutions don't help much at all.