You only need high speed if you shoot long continuous sequences of images and need to be able to clear the buffer quickly ready for the next series of shots. If your shooting style is only one or two shots at a time and then a pause of a few seconds before the next shot then you don't need a high speed card. If you want to shoot video (not an option on the 450D) then you might need a faster card in order to receive the constant stream of video data.
As for the quality of the card, digital being digital, and all just ones and zeros, there is no such thing as a card that stores "poor image quality". It will store ones and zeros the same as any other card. The problems arise when cards stop working for some reason and then all the data can get lost or corrupted, but often can be recovered with special utility software (free).
People say that buying quality name brand cards increases the chances of reliability and that may be true. I've only ever had one dud card, which was DOA and replaced quickly, but I have never had a card go bad on me. I've used brands including Sandisk, Transcend, Integral, Datawrite (the dodgy one), Sony, Viking, Kingston (I think) and even a fake Sandisk from eBay. Nowadays I tend to buy only Sandisk or Transcend but even for my 50D I have chosen to go for a relatively slow 32GB 133X Transcend rather than a vastly more expensive and faster card.
