This HP VH638AT compatible 4GB DDR3-1333MHz Dual Rank X8 Unbuffered 1.5V 240-pin CL9 UDIMM is 100% guaranteed to be compatible with your system and to work right the first time. Adding additional memory is the best way to increase the performance of your system. All our memory is built to strict JEDEC standards to meet or exceed the Tier 1 OEM's factory requirements. Our products are also guaranteed by federal law to not affect or void OEM warranties.