With a 3500 your going to have to use the bus. Just start raising your bus a few mhz. at a time till it gets unstable. Then just back it off a few. If you want to go higher you will have to bump up your CPU voltage. If you do raise you CUP voltage keep a eye on the CPU temp.
When you use the bus your memory speed will increase too. You can either bump your memory speed back or after a point you will have to raise your memory voltage too.