The long/short answer that I know of is just " it depends".
Read this short article for an idea of what I mean:
rsart - Rick Stirling, games artist » Blog Archive » How many polygons in a piece of string?
You can also read this one:
rsart - Rick Stirling, games artist » Blog Archive » Yes, but how many polygons?
I am working of a fantasy MMO right now, and 1000-1500 is fairly standard (since you want to reach a wider target audience with lower system requirements). I would say that for a MMO with direct X 9, 8000-10000 is too much. Again it really depends on the style that you are going for.
Based on what little I know of what you are doing, i would go for around 1500-3000. Just my opinion though.
Good luck.