Is there a way of making the 8GB card bootable from a PC?
1) There are a few things to consider when formatting your SDHC card. In addition, since the SX20 IS is still in beta, you won't be able to install CHDK automatically yet, so a while ago I created a Windows tutorial that will help with the formatting & CHDK installation. After another SX20 poster mentioned a new formatting possibility (dual bootable partitions on a 8G card), I updated the tutorial to include it as well. So far, I've found dual bootable partitions on a 8G card to be the most useful for me. Here is the link to that discussion: http://chdk.setepontos.com/index.php/topic,5344.msg51806.html#msg51806
Here is a direct link to the Windows tutorial: http://rapidshare.com/files/400436852/Installing_CHDK_on_SD_Cards__Windows_OS_.pdf
Just use the 'Free User' option to download. Since I'm still using Vista, I haven't tested all of the apps mentioned on a Windows 7 yet.
2) Since formatting an 8G SDHC card will take time, be sure you start with a SDHC card that will meet the capabilities of the SX20 IS. Not everyone is aware that there are different classes (speeds) for SDHC cards. For example, in order to use the SX20 IS at it's fullest capability, you should use at least a Class 4 SDHC card. While you can use the cheaper Class 2 SDHC cards commonly available, you may see noticeable degradations in the HD movies and the burst rates when shooting in burst mode. Also the higher speed classes can considerably shorten the transfer times when tranferring photos to your computer. While a class 10 SDHC card is better than you'll need for the SX20 IS, I've read that transferring a loaded 8G card to the computer can be shortened by 10 minutes over a Class 2 SDHC card. The Class of the SDHC card is usually displayed as a number 2, 4, 6, or 10 in a three-quarters circle on the face of the SDHC card. Here's a good summation: http://www.digitalcamera-hq.com/articles/digital-camera-memory-cards-finding-the-right-card
Hope this helps - Good Luck!