Around 100ms is a little laggy, but would probably be playable for most if not all casual players. I'm guessing that the majority of players would be in the US, but there are also a fair few EU members who'd play too.
If you get time, can you give me the times for:
webfusion.co.uk
bluesquaredata.com
globalswitch.com
cheapvps.co.uk
fsckvps.com
pingvps.com
I get low pings with all of the above except for cheapvps.co.uk and fsckvps.com. There should be a few US-based datacenters in the list with a bit of luck - but obviously there's a time penalty involved in sending data across the channel, so pings to US from EU are slow and vice-versa. This isn't a conclusive test of performance, the only true way to test would be to give one a try for a month and see if it works or not. Some companies may host their websites outside of the datacenter they host clients on, so pinging may give a bad result for that reason too.
If anyone knows of a decent VPS provider elsewhere that would be suitable, feel free to suggest them. I don't know if 80-120ms is acceptable for casual play or not (I'd hope that it would be).
Based on results of those pings and other people's suggestions, I hope we can find a host with low ping times for most/all members who want to be a part of this.