250 watts

Hey guys, I've seen a few posts about this but they don't really satisfy my question. I just bought an old Dell Optiplex 390 (Desktop version (DT), so not full size). I want to add a graphics card without having to upgrade my Power supply (standard 250W). I only want it to speed up everyday use...

I might build a computer in the near future and want to overcIock it, but I don't usually have that much time to run stresstests for hours. What's a decently safe setting for overclocking my processor with my cooler? Here's the parts list http://pcpartpicker.com/p/JD2v99 Thanks in advance.