In reply to squicky:
Some thoughts from going to the Valley this autumn:
The rock really isn't like anything you get in the UK. Can't say for Valle de Mello, but it doesn't equate to UK routes at all. Yosemite feels weird and alien at first, but you do get used to it with practice. Honest! Being able to climb cracks here first is vital.
Full body fitness is vital, almost as useful as the typical Brit climbing wall fitness. You want to be strong all over as there's lots of high stepping, palming and general yarding about.
Get good shoes. I found I couldn't wear my usual UK ones, partly from the heat (it's a hot place) and also because they were agony in cracks. Get a pair of specialist crack climbing shoes.
I think if you've three or four weeks you'll have enough time, depends what you want to do too, a wall, or long free(ish) routes, or cragging.