Hey Langley - I had a 8500 in my old Mac and when I bought my DP 1 GHz I swapped the 9000 in it for the 8500. Though the 8500 is a little better card, it is a slight advantage over the 9000, but nothing you would really see or notice in everyday non-gaming computing and/or even gaming. I'd go with whatever you can get the best price on, but if games are a big concern, maybe you should go for a newer card?
There are a few posts out on this issue - and I remember finding some sites that list the one or two differences between these cards. I'd give you some links, but I have to run! Also, call ATI - they seem nice enough and know way more than I do! (not hard to do).