I have many years of experience working with and studying monkeys both in captivity and in the wild. I've worked extensively with ex-pet monkeys and have studied the pet trade and animal welfare extensively. I hope that you will consider what I have to say.
It is not a good idea to get a pet monkey for a variety of reasons, which include your health and your safety; the welfare of the monkey; and the negative influence having a pet monkey will have on the survival of threatened monkey species in the wild.
No amount of effort or good intention on your part can change any of these problems. The fact is that monkeys are wild, not domesticated animals, and they will act accordingly. (It is important to know that being born in captivity does not mean domestication - domestication is a measurable biological process that happens over many generations of selective breeding). Even small monkeys can and do become dangerously unpredictable. Do you know that capuchin monkeys, which are about the size of a housecat, regularly hospitalize people, often disfiguring or disabling them for life? Some monkey owners will tell you that they have never been bitten - but if it it true it probably means that either the monkey has not yet reached adolescence (when the problems start... which may not happen for a few years) or that their monkey is so damaged, either psychologically, or physically, or both, that it is not acting like a normal monkey should.
All monkeys are intensely social animals that are hard-wired to live in complex social groups and to range large and complex territories all day long, every day. When they are denied the opportunity to do this, they suffer - and you can see this in their behavior. The problem is, that very few people that own monkeys have any idea what 'normal' behavior is, and so have no idea that their monkey is acting very abnormally!
To be kept as pets, monkeys are taken from their mothers as infants (and mom is just impregnated again as soon as possible to produce another baby to take away). Maternal deprivation is well-known to cause lasting psychological damage, and even sometimes hinders proper brain development. Human caretaking is no substitute. How can a trade that is founded on inflicting this on intelligent, emotional creatures be anything but wrong?
Plenty of pet monkeys are born in captivity so their owners tend to think that there is no relation to their pet-keeping and the capture of endangered species in the wild. But this could not be further from the truth. anybody working in the field knows that when people who live in primate habitat countries know that Americans and Europeans like to keep pet monkeys, more monkeys are caught (which includes a lot of killing, too) and kept as pets, used as photo props, and used in attempts at smuggling.
You must be an animal lover if you are interested in monkeys. Please, be realistic about it - keeping a pet monkey is a bad idea all around. I hope you will make the right choice!
(p.s. in response to the post that says the professionals feed monkeys on chow.. this is not true, and another huge issue for pet primates is that their owners almost always feed them inappropriate diets - hence high prevalence of diabetes and other nutritional diseases... and if you keep a monkey in a $500 enclosure there is no way that you are providing it with anything even approaching a suitable environment)