Am looking at either an iphone or htc desire, orange have a tarif for the Desire which has a 500Mb limit, how much browsing is 500mb? Is it easy to go over this just with emails, browsing this forum and watching the odd youtube clip?

For usage like your describe 500mb will be more than plenty*. I have used 429mb doing pretty much as you describe and that has been over several months.

*Assuming that is 500mb a month.

Whichever you decide to get, there are plenty of apps you can download to help you keep track of your usage. As long as you aren't streaming YouTube over 3g all the time you should be fine.