General advice I gather for setting QoS properly is to enter the upload and download bandwidth to 80% of the best speed you have from a broadband speed tester. To get as accurate of a figure as possible test directly by plugging into your cable/ADSL modem.
Hope that helps.
What firmware are you running? More recent firmwares, including the latest 260, have a problem with wireless stability on 2.4 Ghz WiFi for some users that may be making the problem worse.

Welcome To SNBForums
SNBForums is a community for anyone who wants to learn about or discuss the latest in wireless routers, network storage and the ins and outs of building and maintaining a small network.
If you'd like to post a question, simply register and have at it!
While you're at it, please check out SmallNetBuilder for product reviews and our famous Router Charts, Ranker and plenty more!