1. When a checkpoint is reached all blocks that do not belong to the best chain are removed forever from the block chain (or from the inventory)
No need to delete it from the blockchain
2. All block hashes up to the last checkpoint are stored inside the Satoshi client so that no unknown block can be sent.
225000 * 32 bytes = 7200000 bytes = 7.2MB
Not sure it's worth it as 1. is enough