This is very personal, so the only true way to figure this out is in practice. Some providers provide some guidelines about how much data is used when you do something, and give advice which subscription to apply for which usage. But be aware this advice comes from same folk who are trying to sell you something; hence may be skewed.

I have FUP (not allowed to regularly use more than 1000% of average data usage). While the FUP is not transparent (I cannot easily calculate my total usage from telco website nor can see average monthly usage) I am otherwise OK with the FUP and never received complaint. I use about 300 MB a month with 24/7 e-mail & RSS, with sometimes also web browsing and IM. When I am near WLAN for which I have access (800+ in NL, home, and some others) it automagically will use that connection. I use offline maps.

If data is expensive make sure you make precautions. For example, use a proxy (by provider or on Maemo or e.g. run by Opera) in your browser which downsizes the quality of images. Because N900 has good screen quality you may opt for not doing this or only when situation requires e.g. when on slow network, with very big pictures (e.g. 1+ MB), or when data is very expensive, or when roaming. There are also widgets you can run on desktop which show your data usage in development, such as here.

Statistics show people using iPhone which heavily uses GPRS use more data than people with smartphones less optimized for internet usage. Because N900 is not primary a phone but rather a mobile computer you can expect the same from N900; so the data usage of N900 is probably more than what I described here above (Nokia E71).

Once Nokia N900 is out, we can perhaps gather some statistics from various users, and see if any users got complaints from their ISP.