Unless you are using advanced features or a seasoned flier all you will probably notice is better graphics between G3 and G4. G3 is upgradeable to G5 for under $100 though. G5 and G4.5 are know to have several problems on a variety of hardware that meet their listed requirements.
G3 will run almost any PC. To get the better graphics and performance out of G4.5 and G5 require a high-end gaming computer ($$$$) and high-end video card ($$$$).
I had G4 and liked it but have found that the free program FMS meets 90% of needs . I fly slow WWI planes, no 3-D or aerobatics etc.