Usually a hardware upgrade isn't required for a new operating system. Yet if you are planing to upgrade Microsoft Windows XP to Microsoft Windows Vista, it's almost impossible to avoid a PC overhaul. Aside from things like the speed of your processor (minimum 800MHz), and using a videocard that supports Vista's sublime Aeroglass graphical interface (DirectX 9), the most important and limiting factor is going to be memory. If the PC doesn't have enough RAM to satisfy Vista's intense thirst, you'll be the slowest thing on two wheels. For the record, Vista's minimum memory requirement is 512MB, though realistically that should be doubled.
Not sure which DDR memory your system takes? Here are some tools to help you find compatible DDR Memory.
DDR3 is the next-generation, high-performance solution for CPU systems. DDR3 memory are twice as fast as today's highest speed DDR2 memory products. Select your DDR3 Memory from the following.
DDR2 introduces some new features which allow it to ramp up to much higher speeds (with correspondingly higher bandwidth) and higher memory densities, all the while using less power. DDR2 memory uses a new form factor, a 240 pin DIMM (Dual Inline Memory Module) which is *not* compatible with current DDR memory slots. Upcoming chipsets by Intel and other manufacturers will support DDR2 specifically, and are not backwards compatible.