I use everything in my 40D - from cheapies to the expensive stuff.
I'd get 4GB cards or higher, personally.
I have and love the Transcend 8GB cards - and they are $36! Can't say enough about these cards. There is no real life difference that I can see between these and the Extreme or Ultra SanDisks that I also have.
Right now SanDisk has a rebate thing on and it brings their 8GB cards down to about $36, too, when you buy three. The rebate is not cash back, but a Visa thing to use wherever Visa is accepted. I just ordered some of these the other day.
Frankly, you'll hear a lot about which brand of card to buy, but I have yet to notice any difference in camera or in my card reader...my main three are the Transcends, SanDisk (but I will not pay more than double what the Transcends are) and the Lexar Pros.