4GB Memory Upgrade for Dell Vostro 400 Desktop PC 4 X 1GB DDR2 NON-ECC PC2-6400 240 pin 800MHz DIMM RAM (PARTS-QUICK BRAND) at Amazon.com
Dual Monitor Power Combo Set - Dell Vostro 400 Mini Tower with two 19" LCD Monitors - e198wfp. Vostro 400 Mini Tower, Intel Core2Duo E4600 (2.40GHz), 4GB RAM, 128MB ATI Radeon HD
Best Buy: CMS 2GB (2x1GB) RAM Memory Compatible with Dell Vostro 400 Mini Tower Dekstop DDR2 CM128646400DIMMX20202