As Hugh says, for astro it should not matter what speed you get.
Assume you might want to be able to shoot long bursts. Take the fastest frames/sec the 350d can shoot (ie 3), multiply by the size of the images (RAW+JPG) in megabytes (about 8+1.5). That comes out at something like 30MB/sec. SanDisk Extreme would be fine. Ultra would be marginal. A 16GB card should hold about 1600 RAW+JPG images.
You're lucky. Extreme Pro get no-where near keeping up with the burst rate of my a77.
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