well , if you planning to get an landphone by SLT (Sri Lanka Telecom) you can get ADSL 512kpbs or 2MB connection for 15US$ -50$ per month , connection and speed is reliable,
if not you can get mobile 3G connection with HSPA modem the price would be almost same but 3G coverage is not there when you go out from city then it will switch to GPRS which is pretty slow.
so the best thing is go for ADSL , its fast and no bandwidth cap