First if you can upgrade your SUSE version, it will be better to get security updates on line, the last release is 10.1 .
The 9.0 release is no longer supported.
Second if you can buy an ethernet modem/router instead of the Sagem Fast 800 USB modem, it will be easier to install and configure it under the Linux OS. You may have to buy also a NIC, if you don't have one available yet on your machine.
If all this is not possible for you, you will have to use the eagle-usb 2.3.3 driver on your that old SUSE Linux release.
But if security is not an issue for you, this will work nevertheless.
You will have to first install the kernel source rpm package, the same as the running kernel (uname -r).
Then go to eagle-usb.org
to download the driver and read documentation on how to install it.
I used this driver for many months and it worked well under the SUSE Linux OS.
Another point to mention is that there exists different type of Fast 800 modems, if yours is the E4 type, you will not be able to make it work under Linux AFAIK. All the other types work with the eagle-usb driver.
Remember that my two first points are the preferred way to go.
In all case I hope you to succeed in your attempts with your machine and Linux.