Thanks for your reply.
I understand that 16 GB cards are available in the market. But the HTC site lists support for upto 4GB only for the HTC P3400i :
SDIO/MMC (Up to 4GB)
But I believe the page has not been updated since it lists the OS as:
Hey guys, I want to buy an SD card for my new P3400i, I want to have lots of mp3s and vids, so would like to know what's the maximum storage capacity it can handle. Also does it support SDHC??
Thanks in advance.