This book has popped into my head and I'm desperately trying to remember what it is.

This book must be from the 80s or early 90s - about a games developer who is pretty average and makes 'ok' games. He meets a stranger who has a computer with 'a megabyte' of memory (which for the time is super-advanced and expensive, considered not mass market), who shows him a really awesome game developed on the system.

I'm fuzzy on the details, but it transpires this stranger is somehow the guy's future self. I can't remember how it ends, I want to say the guy has a dream about his future - he meets a girl? But also somehow it's post-apocalyptic and there's been a nuclear war (or something) that the guy may or may not be responsible for somehow.

Sorry, that's the best I've got. Help!