depends on ur video card and cooling situation. they just the recommended boundarys. basically, u should overclock small bits by bit if u hav no idea how good ur card overclocks. if i was u i would go to a hardware forum and ask to save time testing. never overclock too high if ur unsure. it can overheat and burn ur card. thats why ur meant to increase the overclocking bars in small amounts, then test the card for stability, and continue overclocking. if u start overclocking too much, windows might freeze, or u might start getting artifacts appearing on the screen. if thats the case then just reduce the clock speeds a bit till its stable again.
for instance, my msi geforce 4 ti4200 (64 megs) is now clocked at 315 core 600 mem, whilst the default is 250 core and 515 mem. this is pretty much my limit. any more will start causing issues. also i hav 2 extra fans in my case, so hopefully things stay a bit cooler.