Both hunches of yours is where I would start. The best advice I can give you at this point is dont get snockered with a generic crap PSU.
I would install the new case switch (cheap fix) best buys or newegg will have those.
This is the PSU I would get without a moments heistation for your system.
http://www.newegg.com/Product/Produc...82E16817104953
six years on a PSU is rather impressive; also have you ever upgraded the video card from the original ? and if you have; what make and model did you install?
keep us posted on your progress.