Originally Posted by Ballpointpenner
Thanks for the indepth answer, JimC. Really, I don't need sport-shooter performance and I've never used bursts in my life. Basically, I just want to be able to shoot a consistent run of 1-2 shots per second for a few seconds, put the camera down for a few more and then go in for another round... nothing super-sonic, here. Would the Xsi/Xt1 be able to deliver that?
Almost any modern dSLR will do a bit better in that area compared to your Pentax. Your camera only has a 3 frame buffer shooting raw (the amount of space available in faster internal memory), with very slow write speeds to memory cards (it needs around 3.6 seconds for each raw file, regardless of card speed). So, after 3 shots, your frame rate is going to slow down to the speed it's able to write to the card (approx. 1 photo every 3.6 seconds).
If you take a look at the review conclusions here at Steve's (last page before the sample images in each review), you'll usually see some performance tests with card speeds a typical user may have. For example, you'll see these results using a Sandisk Ultra II (a popular 66x card when that camera came out) when shooting raw:
Shooting in RAW mode slows things down a bit. The Continuous burst mode captured 3 images in 8/10 second, with subsequent shots at 3.6 second intervals as buffer contents were processed; it took about 10 seconds to empty a buffer full of RAW images.
If you check the K100D review at dpreview.com, you'll see that Phil got slightly slower results when shooting raw using faster Sandisk Extreme III and Lexar 133x PRO Cards.
IOW, your K100D's speed is the bottleneck, not the speed of the memory card once you get into reasonably fast cards.