Frequently that speed is a theoretical maximum - not an actual - and as with
anything if the lines are busy, it will be slowed down. The speed limit on
the highway may be 55, but if there is gridlock, you obvioulsy won't go 55.
Also, have you reset your modem lately? Sometimes that helps.
Do you have a spliter on your line at some point? Either inside the house
or outside? That would make a difference.
550MHz is somewhat slow for a CPU too though. If you can, an upgrade would
help.
526MB should be enough, although that is an odd number. If your memory is
mismatched inside, that could make a difference.
If you have a really slow hard drive, that could make a small difference.
Others have recommended speedtest sites. Here is another:
http://www.bandwidthplace.com/speedtest
(can only be used 3 times per month).