Go for a P5Q motherboard if possible. The P5Q series are pretty OC-friendly, but I think they're incompatible with Nvidia cards. How much more (or less?) do you have to pay to switch the 9500GT for a Radeon HD 4850 or 4870?
Also, in practice, a 420W power supply will most likely not provide your components with as much as 420W of power. Your choice will most likely be sufficient, but upgrading to a 500W one still seems feasible. Corsair is a pretty good PSU brand.
"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram
Threaded Mode