The XSi is capable of utilizing the speed of faster cards. So, for example, something like the Sandisk Extreme III series cards will give you the fastest camera operation. I wouldn't buy anything slower than a Class 6 rated card (20MB/s or faster)

So, for example, the 8GB Sandisk extreme III would be a good choice at around $47. I have a bunch of Sandisk CF cards and they've all been great.

I've always been quite pleased with Kingston cards and I use several of their cards myself. You can get a 16GB Kingston 133x SDHC card for ariound $40