I currently have a sandisk in my Treo, but as long as its namebrand, don't sweat it! Buy on price, but make sure its a name that your buying. Prices are falling all the time, and the Flash memory market is a very competitive one. I promise, you can't tell the difference between a 1 gig sandisk, and a 1 gig _insert a namebrand here_.
There is solid advice above when it comes to ebay, as far as black market and knock off cards. Buyer beware!

On a totally off topic note, I believe that Flash memory will appear as computer hard drives within the next 3 years! Low battery drain, almost instant on, and almost large enough to run Win/Mac OS, and other apps!!! The applications are limitless!