The xw4550 and xw4600 are desktop workstations with enormous power. The first one is an Advanced Micro Devices machine while the latter one is an Intel based system. They are designed and engineered to give you and your business a low cost professional edge. Both features a quiet, tool-less chassis and clean interior for easy access and are designed for system flexibility and upgradeability options for the future. The HP xw4600 introduces dual PCIe X16 Gen2 graphics interfaces, delivering up to twice the performance bandwidth potential of previous graphics interfaces while driving multiple displays without compromise. The xw4550 can be configured with AMD Opteron processor and up to 4GB RAM. Whereas the xw4600 supports Intel Core 2 Quad / Core 2 Extreme processor and 8GB RAM.

Both of them provides two 3.5" hard drive bays (internal), dual layer DVD burner, 1 PCIe x16, wide range of Nvidia Quadro graphics card, and Vista.

Source [xw4550]
Source [xw4600]