Just as Canon has decided to equip the Canon EOS-1D Mark lI and the new EOS-1Ds Mark ll
with Dual Card Slots, (using both CF and SD memory cards), the speed, and more importantly, the capacity, of these types of memory devices have recently increased considerably.
First, a few definitions and clarifications that might be helpful:
Compflash (CF) cards are available in capacities from 16MB to 12GB. The CF Specification can actually support capacities up to 137GB. The only difference between CF Type I and CF Type II cards is the card thickness. A CF Type I is 3.3 mm thick and the CF Type II cards are 5mm thick. A CF Type I card will operate in a CF Type I or CF Type II slot, however, a CF Type II card will only fit in a CF Type II slot.
The Microdrive is a type of CF Type II card and while not flash media, these devices are extremely popular with professional photographers because of the inherent security that this type of media offers. These devices are essentially miniature hard disk drives.
Here are some of the option when looking for high capacity cards:

The Lexar Media 80X Professional Series CF cards are available in capacities up to 4GB. These cards are among the fastest available, and are often the card of choice for the professional photographer.
Claimed by many to be the fastest high capacity CF card on the market, the SanDisk Extreme III is backed by a no-questions-asked lifetime warranty and also come with the data recovery software, (RescuePRO), a dedicated technical support line and a travel case.
A few camera's in the Canon lineup are unique in regards to memory cards:
First, the PowerShot Pro 70 contains two CF card slots, a Type I and a Type II slot. Secondly, as mentioned above, the EOS-1Ds Mark II comes equipped with two card slots, one for CompactFlash (CF) and one for Secure Digital (SD) Memory cards. The camera can be set up to record images on either card for maximum storage capacity, or, for extra security, the same image can be recorded simultaneously on both cards. And according to Canon, Type I and Type II CompactFlash cards with capacities up to and beyond 2GB are supported on this camera.
The largest (but not the fastest) CompactFlash Type I card is the 8 GB SanDisk Ultra II.
CompactFlash Type II
The largest CompactFlash Type II card is Lexar's new 8GB 40X. This card requires the host camera to support the FAT 32 file system and while it is not the fastest card, it certainly has one of the largest capacities.
Hitachi Global Storage Technologies Inc. (HGST) has recently announced that it will begin to ship a 6 gigabyte Microdrive called the 3K6 (named after the drives speed of 3,600 RPM). This drive will hold 50 percent more data than the company's current highest capacity model, it said Wednesday. The new drive will cost US$299, which is a 40 percent reduction from the $499 price at which HGST has introduced new Microdrive products in the past, said Bill Healy, senior vice president of product strategy and marketing at HGST. The company is also lowering the price of its 4GB Microdrive to $199.
Customers who buy HGST's drive in bulk for use in their own products have been enjoying regular price cuts for sometime, said Healy. HGST's customers include Apple, which uses the 4GB Microdrive in its iPod Mini music player and on Wednesday announced a 6GB version of the player (using the new 3K6 drive).
Can I use the Microdrive in my Canon digital camera?
All
PowerShot digital cameras, and all EOS digital SLR cameras, use the
CompactFlash(CF)-format memory cards to save your digital photos.
Compatibility,
therefore, is dependent on the physical size of the CF card slot in the
camera. Following is a list of models which are compatible with Type II
(Microdrive) devices:
PowerShot 600 (with optional PCMCIA adapter)
PowerShot S10
PowerShot S20
PowerShot Pro 70
PowerShot Pro 90 IS
PowerShot G1
EOS D30
Sony also offers a 2GB and a 4GB CF Type II card using the Microdrive format.
Also, Seagate has now entered the market for Type II cards, offering a 2.5GB model and also a 5GB card as well.
To best optimize the enhanced speed provided by cards such as these,
your camera needs to have an optimized CF interface. Canon cameras
with this include the EOS-1D Mark II, EOS-1Ds, 20D and 10D.
Performance increases while using these cards are most noticeable
during data downloads using the USB 2.0 interface.
Check out this URL from the CompactFlash Association website for more information about CF cards.