I'll be into my summer job again soon and I have some ideas of what lens I'm going to pick up for my D40; a 50mm f/1.8 and a 55-200 VR lens. My budget is under $500 CDN. Honestly though, I can probably do without them for the summer, but I just want some new hardware and have some fun looking forward to something in the summer.

Are there any other offerings from Sigma or Tamron that is cheaper than those lens?

Also another question. How often do you guys clean the dust off your sensors?