my internet constantly gets dropped, please help

Ok when im downloading something, or heavily using my internet (3 xbox 360's attached to my router for example), it just shuts down and i have to reset it.. o my god its annoying. I have adsl, from telus.com (just regular highspeed). Im not sure why i dont just go with cable but o well. Anyways so im fairly certain it has to do with my old router and modem. I have the D-Link DSL-300G Generation 2 modem. My router is the D-Link Ethernet Broadband router. I was thinking of upgrading to this http://www.ncix.com/products/index [...] _id=21730. My question is, does this thing count as a modem and router?? and if so is it smart to just use it by itself without another router, because routers help to block hackers etc..

Also i have a laptop i sometimes use in my house, do i have to buy some card or anything for the laptop to use the wireless?? Also how can i stop other people from using my connection?