Well, I use 4Gb and 8Gb memory cards and I rotate them (I have about 10) every time I do a full day of shooting -- more often if I do something critical like a wedding.
How many you can get on a card depends on whether you shoot RAW or JPEG or both, and what level of JPEG you use.
I shoot RAW, and a 4Gb is enough for me before I switch to the next one.
As to the LCD covers (I think that is what you are talking about) -- I removed mine within a few minutes of getting my D90 -- I kept it in case I changed my mind, but it does look clunky and seems a bit overkill, so away it goes. I do clean my LCD screen with an LCD lens-pen every few weeks -- it does have a few minor scratches, but nothing that bothers me!