Soleron on 08 January 2011
Given your circumstances, I think a prebuilt will be cheaper as you say (those video card prices are insane).
So the first computer you mentioned with a GTS 250 or HD 4850 or HD 5750 or GTS 450, whichever of those is cheapest. You don't need to replace the power supply; 450W is enough. The rest of the components are fine.
I don't think you need an Intel processor for PS2 emulation? AMD is compatible with all x86 instructions except SSE4.x as far as I know.
