Don't go cheap-o. I bought the 19.99 one that was posted on these forums, linked to an ebay auction. Too good to be true. It said SanDisk on it however the paint on the card rubbed off, so I know it wasn't sandisk.
On my Original Droid, it would randomly say card damaged reformat was the only option. The good news however is that I could still read it with a card reader on my computer, so I could get my stuff off and transfer it over. Either buy legit or you may have issues.
Like I said before 90% or more of the cheap Sandisk cards on ebay are Fake. You can scan the card-both sides-and email it to Sandisk and they will tell you if it's real or not. If it's not, get your money back, report the seller to ebay, paypal, whatever. Sick of these scam artists.
Buy it from a big retailer. Amazon, Best Buy, etc. Use a credit card.