Well I don't have any exact figures but I think most MMORPGs like WoW that have fixed downloadable patches (rather than dynamic patching on the fly) will be using somewhere in the region of 10 MB per hour of data transmission. This figure will be a lot higher for FPS games as much more data needs sending between the server and you. I'm not sure about DDO but would think it'd be similar to WoW, in the 10 MB / hr ballpark.
So, leaving aside patch downloads, a 2 GB limit gives you approx 200 hours of gaming. Should be plenty for all except the horribly addicted
Of course that assumes you do nothing else with your net connection. Trouble is there's always games patches, Windows Update patches, web surfing, those annoyingly huge joke email attachments people love to send and so on. So in reality your 2 GB probably won't go as far as you'd like. But still should be able to get a fair amount of mileage out of a 2GB limit. Just make sure you have an upgrade option to a less restrictive package if it turns out you need it.