Adorama has the 1 GB Sandisk Extreme III Compact Flash card for $57.95 + $5 ship, then $15 MIR from Sandisk, $47.95 final cost. Some other places have it listed cheaper but after shipping, tax, no rebate, etc the final cost is higher. NY and NJ pay tax on Adorama purchases.
Ron Galbraith has a chart showing the relative speeds of CF cards using a Canon Digital Rebel XT. This card is the fastest.
Rebate link rebate expires 9/15
If you go higher to a 2 GB , the cost per MB goes down.

a little heads-up about SanDisk extreme III cards: it has been announced that they will be bundled with a raw image workflow software "Capture One LE" by PhaseOne, which is listed at $100. I talked with SanDisk customer service yesterday, and they don't have the bundles yet, but they should be available "soon". They said that they will not give copies retroactively. So, if you need the card now, and you are interested in this software (which is highly rated, but I haven't used it myself, so I cannot comment on it), you better wait (or buy another card later).

avatargod3k said:Actually, the Capture bundle is with the Extreme IV (4), which is twice as fast and was just recently introduced. Nevertheless, i'd recommend getting the 2gb card as it's barely $20 more.

from PhaseOne's website's opening page:

CAPTURE ONE & SANDISK EXTREME III CARDS
SanDisk® the world's largest supplier of flash memory data storage products offers Capture One LE RAW workflow for FREE with all SanDisk Extreme III CompactFlash® cards.

maybe that is wrong, but that is their info.