Dimension 4700C - Power Issues

Two weeks ago I came back to my Dell Dimension 4700C and it was mysteriously off, with the flashing amber light.  The machine would not power on.  My 2 year warranty is now up, but I paid the one-time troubleshoot fee to Dell technical support.  They walked me through the troubleshooting, unplugging internal components, etc.  The final diagnostic was that it was either the power supply or the motherboard, but they could not say 100% which one.  They suggested buying both and seeing which one worked, and returning any part I didn't need. 

 

I ordered a replacement power supply (new) from a place I found online that had it (curiously, the Dell spare parts dept. refused to sell me the part -- the guy said I should take it to a computer technician.  I didn't have time given work right now so I just ordered the least expensive one for now).  So I replaced the power supply and miraculously, the computer turned on, green light, seemed like I fixed it.  This was 2 nights ago.  After turning on, the first screen prompted me to run setup and reset the date and time. 

 

Today, in the middle of running iTunes, the computer just shuts down completely.  One second on, next second everything just black.  Amber light.  I was able to turn the machine back on, but I am wondering if I am going to have a continuing issue.  I was again prompted to run setup to reset the date and time.  Then my girlfriend tells me that just earlier in the day, she had to press the power button a lot to get the machine on.  Also, since I replaced the power supply, I can tell that it sounds different.  It sounds like the fan is having to work a lot... before, it would rev up like a car.  Now it's intermittant.  Also, when I run programs, surf the web, etc., it sounds like it's having to work pretty hard. 

 

Long story short, I'm not sure if there's something else wrong with this machine, if it's on its last legs, and am wondering if it's going to completely putter out on me.  Anyone have any bright ideas?  Much appreciated.  Not sure what else to do.