Power supply so i need a new power supply for my m7250n pc now the old power supply is 300w can i buy one that is over 300w and would it still work also would it be a good idea to get new case for it too

Is it worth it? a new PSU shouldn't cost you more than 20 euro/ $ whatever part of the world you are in. Unless it has the old 3.3V conectors as well (which would be surprising) in which case find an old 2nd hand one.
If it is making a lot of noise it is probably failing due to dust and/or overheating and the rest of the PSU can't be in great condition either. Sometimes the non-eviromentally friendly option is the best one if you leave the old PSU for recycling , more so.
Good luck.

20-60 Dollars
A great site to buy from is HERE
NOTE: all these power supplies will work for your computer, you may want to pay attention to the dimensions though

Acquire a replacement power cable simialr to the one missplaced.

your power supply is faulty you will need to replace it. If you recently installed a new hardware it could be you don't have enough power to, either upgrade you power supply or remove the hardware.

There are many power supply calculators out there. As long as you know what the components are on your computer, you should be able to find max power usage by inputting all the components. Google search terms:
power supply calculator.

You hit that one right on the dot.  ATI website showed the specs for that graphics card you installed as needing a 300 Watt power supply.  Good news though... I researched newegg.com and I believe I found your 300W power supply.  There are 4 different Optiplex gx280 designs with 4 completely different power supplies.  Since you bought the low-profile video card, I believe you have the slim computer case and this power supply should work for you...
http://www.newegg.com/Product/Product.aspx?Item=N82E16817374002

Good Luck

The computer has a 300W powersupply from factory, I would buy a 400W powersuplly for it, depending what components it has inside, if the video card is upgraded to a new one 300W won't be enought, and in that case even 500W might be a good choise. if its with the original parts from the factory it should do fine with either 300W or 400W. ATX here are powersupplies: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=40000058&Description=400W%20ATX%20power%20supply&name=Power%20Supplies note that if the link isn't clickable then select it completely, copy and then paste it to your addressline to open the page.

hold the start button in and check the back of the case to see if the power supply fan turns while the power button is depressed. If it turns while the button is pushed, but then stops when you let off the button, then the power supply is probably bad and needs replacing.

Hello, you can find them on ebay, If you get a new one and it does not work, the moterboard could be bad. 2 things, the power box or the motherboard.

Ivy My field is in home appliances. I do fix my own computers and have replaced a few power supplys. The only thing I could say to you is go back and recheck your connections. They should only fit on the bord one way. The other connections for the hard drive and cd drives and so on really dont matter. so good luck and I think if you recheck it all the computer will work.