You should first read the terms of your agreement with your ISP and find what data rate they guarantee.
I don't know about yours, but my Spanish 256Kbps service guarantees just 10% of 256.
I hasten to add, that whenever i test it, it averages out at 190-200.
You should also be aware that downloads can be affected by lots of factors other than the maximumbandwidth on your line - like how many are on-line, how busy the server is, some degree of overhead all the time (ie a 1Mbps service will never download at an average of 1Mbps, as there are return packets, retranmissions, rundundany error checking bits, etc.)
You will also find that some sites simply won't supply you data at that rate under any circumstances.