How much memory capacity is enough when you are travelling? 

As digital cameras capabilities skyrocket in megapixel count and plummet in pricing, this question becomes more pertinent.  Luckily HyperDrive seem to have a ripper of a solution for this conundrum with the HyperDrive Colorspace.
 

The Hyperdrive Colorspace is a compact portable device that backs up a memory card with no computers required.  The card support reads like a who's who of flash memory, and a high resolution 2.2" colour LCD is capable of displaying images in JPEG or RAW format on the fly as the backup is happening.

The entire product is designed to be easily upgradeable, with support for any 2.5" IDE/PATA hard drive and no capacity limit.  The Lithium Ion battery is capable of backing up 120 Gb of data on a single charge.

Specifications:

Dimensions: 133 x 75 x 25 mm
Weight: 270 g
LCD display: 2.2" 262K colour TFT LCD
Resolution: Hires 320 x 240 pixels
RAW support: Nikon NEF, Canon CR2, Fujifilm RAF, Olympus ORF, Pentax PEF, etc
Card support: 3 slots supporting 18 memory card types: CompactFlash Type I/II, MicroDrive SD, SDHC, MMC, xD, xD Type H/M, MemoryStick MS, MS MagicGate, MS Select, MS Pro, MS Pro MagicGate, MS Duo, MS Duo MagicGate, MS Pro Duo, MS Pro Duo MagicGate
Backup speed: 50 Mb/s (max), 25 Mb/s (sustained)
Computer interface: USB 2.0 High Speed Mode via UDMA66, USB 1.1 backward compatible
Battery: Rechargeable Lithium Ion 18650 (3.7 V, 2200 mAh)
External battery: Optional 4 x AA external battery pack
Power: AC Power Adapter (5 V, 900 mA), USB Bus Powered (5 V, 900 mA), Car Charger (12V DC, 500 mA)
File system support: FAT32
Memory card: FAT12 / 16 / 32
Operating system: Windows 2000 / XP / Vista, Mac OS 9.x, OSX and above, Linux
Accessories included: USB cable, Universal Voltage (100 - 240 V) AC adapter, Car charger, Soft carry case, Screwdriver, Screws and User Manual

Available from here for prices ranging from USD$199 (case only) to USD$459.00 (250 Gb version) and 3 sizes in between.