Yeah the Celeron G530/G540/G550 are actually the extremely powerful for their price. They certainly beat out older Pentiums, and even Core 2 Duos clocked at 3.0Ghz+. They're on par with the Core 2 Quads I believe. and the new celerons only cost $40.
With the regards to the power supply... It is rated for 450W, but I know it can only do 230W. That being said, I have done the calculations and the 9600GSO + Celeron + 1xHDD should only pull a 100something watts. The 9600GSO has less than a 75W TDP (does not require a PCI-E connector) and I was planning to underclock it anyway.
The laptop 2.5" drive should work... won't it?