There are three card-readers available dubbed Neo 2 in 1 (supports Compact Flash), Neo 4 in 1 (supports Compact Flash/SD/MMC) and Neo 5 in 1 (supports CF/SD/MMC and is a AA Battery pack).
The upper limit of storage you can gain with those adapters is still just 4GB. The Sony PSP firmware does not support more. The Datel HDD, available since a while is providing that storage and is better integrated into the PSP. The card readers are dangling on cable. Only the Neo 5 in 1 can be sandwiched onto the back of the Sony PSP. The Sony PSP card readers start at $79.99.
More details and many photos on Lik-Sang.