A 4.5 inch telescope is pretty small when it comes to astronomical instruments, so you right to want to gain more aperture.However you have to think very carefully if you ever want to enter the world of astrophotography.If the answer is no then a large aperture dob would suit your needs.If however you think you will then you really have to buy a scope tailored for this which would generally mean a lot more expense.Let me assume you are going to buy a scope purely for visual use.I think there is a simple answer,buy the largest aperture scope that that you will use regularly.This means if you have to transport it ,it can be carried easily and does'nt take for ever to set up.What can you expect to see with an 8 inch over your 4.5? well an aperture jump of that magnitude will extend the available objects you can view considerably.You will be able to view many more deepsky objects and split closer double stars.The orion nebula will show more detail and M13, the globular in hercules will be fully resolved.Don't expect galaxies to suddenly develop bright spiral arms,though they will appear brighter and be far easier to spot.On the magnification front your 4.5 inch scope I would imagine would be working at the extreme upper limit at x200.In fact working on the old maxim x30 per inch would give a magnification in the region of x135.On your 8 inch you can expect to use x200 comfortably. hope this helps.