In principle, you want to buy the least expensive (= simplest specification) card you can that has been manufactured by a 'name brand' company.
You don't want or need the 'high speed' cards that are made for digital cameras and are capable of recording at very high speeds.
There are 3 different levels of industry specification for CF cards, and the decision about what level to purchase for any particular device depends on how frequently, and at what speed, the device needs to write to the card. Because the GPSR never writes to the card (it only reads from it), the simplest / least expensive / lowest specification card is more than satisfactory. Just buy one that has a recognized brand name, so you have some assurance that the company that produced it has some quality control in place.
Before someone points out that "hey, the GPSR writes to the card when you load map segments" - yes, this is true, but the USB 1.1 interface that is used to get the map segments from the computer to the GPSR is limited, at its maximum throughput, to about 10% of the write speed capability of the slowest specification card.
PanEuropean