First, a warning: The R-D1 will happily take photos without indicating that you’ve forgotten to insert an SD card.
The R-D1 is unable to make use of high-speed secure digital (SD) cards that transfer data quickly: regardless of a card’s speed (cheap ‘standard’ cards versus a Sandisk Extreme III), it takes about 4 seconds to write the data to the card after taking a photo.
However, there is a marked difference in the time needed to download data from the cards to a computer using a USB 2 card reader: downloading 200 MB (~20 RAW files) took 40 seconds versus 20 seconds for the standard cards versus the SanDisk.
2 GB cards
Epson says that the R-D1 can only use cards up to 1 GB. However, some R-D1 owners are using 2 GB cards (various makes) with no problems. That said, there have been reports of users being unable to download all the files from 2 GB cards; anecdotal evidence points to the card reader being the cause of the problem, not the camera.
Also, the R-D1s manual states that the camera supports cards up to 2 GB. As the firmware update for the R-D1 makes the camera operate identically to the R-D1s, an updated R-D1 will support 2 GB too.
4 GB cards
Neither the R-D1 nor R-D1s can use SD cards over 2 GB: The camera’s firmware only supports cards formatted in Microsoft’s FAT16 file system, which has a maximum volume size of 2 GB. Cards over 2 GB in size have to use the FAT32 file system, which is not compatible with the R-D1/R-D1s.
So, which card?
I would choose a fast, but not the fastest, 2 GB card from a brand-name manufacturer; for example, a Sandisk Extreme III. Such cards are now cheap, but still of high quality (and thus very unlikely to fail).