HTC Desire spec states that it supports:
microSD™ memory card (SD 2.0 compatible)
I saw on the Secure Digital entry on wikipedia that there are 3 protocols:
SDXC - how can I know which of them my device supports?
32GB Lexar microSDHC class 10 card. With SD tools benchmark I get 23.7 MB/s read and 9.1 MB/s write performance on CyanogenMod 7.1-RC1. On stock 2.2 ROM it had considerably lower values with read even less than write. The benchmark is run just after reboot to prevent caching mechanisms inflating the real values.