A japanese made es-120 would probably have the Gotoh Mk II pickups, these are really good, PAF style pickups, and in my opinion sound excellent.
Changing cheap pickups is an easy way to make a low quality guitar sound better, but an ES120 is by no means a low quality guitar, that as I'm sure you know, sounds great already.
Changing the pickups will make it sound different, but not neccesarily better, that's subjective, and unless you go for a different type of pickup (i.e. a high output ceramic) the change in tone will be subtle.
Have a look at the seymour duncan web-site, or more to the point, a listen to the examples of all their different pickups to get an idea of the differences between them.
Also, they style of music you play will have an influence on pickup choice, but I'm sure you won't be a metal player with a 335
I personally like the SD Seth Lovers, the Gibson 57's are good too, but the Gotoh MKII sound very similar to both.
There are also a lot of good 'boutique' brands out there, lollar, bare knuckles etc, but none of them will make a dramatic difference to your tone.
If you have some spare cash and are happy with everything else about your set-up, try some new pickups and see if you like them any better.