Granted, the 50D and 40D share the same body and many features; both have APS-C-sized CMOS sensors (1.6X lens factor); and they even use the same BG-E2N battery grip ($190, street). But as if to underscore the differences, the 10.1MP 40D stays in the Canon line ($950, street, body only).
So what's the big change? Aside from more megapixels, it's the DIGIC 4 processor, with ISOs up to 12,800, in-camera vignetting correction, and three sizes of RAW files.
To give you an idea of the processor's power, even though the 50D packs 5MP more than the 40D, its burst rate is 6.3 frames per second, close to the 40D's 6.5 fps. And the 50D shoots at full tilt for up to 90 large Fine-quality JPEGs or 16 RAW files when you use a UDMA CompactFlash card, according to our tests. With an old-fashioned CF card, expect up to 60 JPEGs.