This is some decent ram for cheap, its better to have 4gb of the same brand than to add 2 more gb of a difft brand, sometimes they dont play well together.
Newegg.com - Crucial 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 667 (PC2 5300) Dual Channel Kit Desktop Memory - Desktop Memory
For a HD, I would get
Newegg.com - Western Digital Caviar Black WD6401AALS 640GB 7200 RPM 32MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives
One of the fastest sata drives on the market, and its only 10-15 bucks more than most 500gb drives.