Tjash has some good points.
Try the 32mb card that came with camera ( you still have that one, right?

)will tell you if the camera's still working properly, and if you can download pics from the camera onto your computer with that card, then the problem may be with the 256mb card. Assuming the included card doesn't double-up the number the files -
Card reader is a good idea. (<$20 and much faster than computer to camera connection (it should be via USB), plus it will save you batteries

)
if the big card is OK you should be able to get your pics onto your computer via the reader.
If you're running WindowsXP, you don't even need Canon's ZoomBrowser software, XP will recognize the camera automatically. You will see it in your Windows Explorer directory You can drag and drop your pics into the folder of your choice. (This is also how you typically access a card reader)
Also, try reformatting the 256mb card
within the camera.
If you already know this, I apologize:
Flip out the LCD and turn the camera on in Playback mode.
Press the MENU button.
Nudge the zoom lever once to the right.
With the 4-way control, press down, keep pressing until you reach the card setting (it will say Format in the upper right corner of the LCD screen).
Press SET, arrow right (4-way button) for OK.
WARNING! This will erase all data on the card, so be sure you have printed what you want off the card, or get the data onto your computer from a card reader first (if that works for you).
Please post back with any questions or difficulties.