If you're comparing the "reach" of a P&S with a DSLR with a particular lens, convert everything to "35mm equivalent". For P&S this is usually given, either by manufacturer or on one of the more reputable review sites. For a DSLR, remember to multiply the focal length with the crop factor (if any). With the 450D the factor is 1.6, so a 300mm lens on a 450D has a 35mm equivalent FOV of a 480mm. A few superzoom P&S actually reach farther, BTW.
A cheap zoom lens (55-250mm) will set you back slightly more than $300, less on the 2nd hand market. I don't recommend the 75-300mm (I have it - image quality is only average under best of conditions). The 70-300mm IS is much more expensive but image quality is much better.
DSLR has better image quality than P&S for 2 reasons : much larger sensor area and ability to use better lenses. Lens design is a compromise between convenience, price and image quality.