You really need a much more detailed description or, even better, a photo, for anyone to be able to identify the spider as there are many with black and white markings.
Think how big it was, and whether it was furry? Were the dots all over the body? Were there any other distinguishing marks? It's behavior could also be relevant to identifying it.
You might have encountered a phidipus spider which are relatively common in the west of the USA. They are a type of jumping spider and there are around 60 described species.
Phidippus californicus is commonly found in the south west of the USA, especially in Arizona, New Mexico, Nevada, Texas and Utah. This particular type usually has some visible red on its body too.
Phidippus spiders are usually active from mid-morning to dusk and you're most likely to see them in bushes. They are often observed running along branches, frequently stopping to scan their surroundings.
It's retreat is a slightly flattened tube of silk, surrounded by guy-lines that attach it to the twigs or leaves of the bush.
As for whether it is venemous, Phidippus spiders do use poison on their prey after they have trapped it. But as the prey is usually wasps, flies and other small insects, only a small amount is required. They are, therefore, relatively harmless to humans.
Be warned though, the spider you saw may not be one of these. Black and white are common insect colors so don't go picking one of these up without being sure about what it is.