first of all.... DataOne is not a good choice if you really want to enjoy the surfing. go for other options like sify and rajdhani.
DataOne is good but they have high fluctuation problem. so it is good for home use.. but for download.. worst.
Rajdhani is having good speed and service which will cost you 5000/- for 2mbps (2006 rate)
another service is there which m not able recall the name at this moment.
Check for free space in phone/card, it must be more than you trying to download. It may be problem with some site where downloading starts from java script, but ucweb can download files of any size. The max size I try is 20 MB.