If your spreadsheets are AppleWorks 6 there is no special conversion necessary. Numbers, either '08 or '09, will open them, converting them on the fly. Numbers '08 has limited formulas, so there may be some unsupported content. Numbers '09 has much more, but I believe it is less than AppleWorks.
I suggest you download the
iWork '09 trial & try it for yourself for 30 days.