I think it depends on the condition the bike was in when you bought it. I'm assuming that it was used, since an 8sp cassette puts it at about 8+ years old (edit: oops, forgot about 8sp Sora!) How many miles were on the cassette/chain when you bought it?

If both cassette and chain were new when you started, just replace the chain. The consensus I have seen around here is that a cassette should last about 2-3 times as long as the chain.