Dexter, there are some Russian dictionaries which show you there the stress is. They are called ‘orthoepy dictionaries’. I can give you the link to them. The only problem is, they are not online dictionaries, they are books. So, you will have to buy them if you want to use them and that’s probably not a good idea.
About the importance of proper stress I can say that sometimes it is important sometimes not. For example, if you say “пИсать” - it means to pee, at the same time “писАть” means to write. As you can see, the spelling is exactly the same and there is no difference in writing, the only difference is in pronunciation, in stress.
If somebody knows the online dictionary which shows where the proper stress is, please let us know, we will really appreciate it.