How important is chain speed designation with its compatibility to chain sizes. I have an 8 speed drivetrain and may want/need to get a new crankset, move to a compact possibly, and everything i look at lists 9 or 10 speed it seems. I want to know if I need to get an 8 speed, and what the difference is exactly (in regards to chainrings)

I want to keep it at 8 speed because I can still friction shift with my DT shifters (they are only indexed to 7) and because 8 speed to cheap to maintain and replace.