You can be pretty sure that if Best Buy tells you something it is usually wrong, and should not be relied on as factual. That should not be the case unless what they sold you is junk (which the review indicate could be the case: http://www.amazon.com/Qwest-Actiontec-PK5000-Wireless-Router/dp/B002IVVBDE),
in which case I would return it during the return period and buy a better model online at a lower cost -- before I returned the old one if you have enough time.
Recognize however that your combo is only 100mbps wired and G wireless, so if you are connecting wireless at any distance it may well be slower than your DSL Internet speed should be. If it is slower than it should be on a wired connection the issue could also be with your connection, unless you have tested it with a different modem previously.