So, when I click the link that you posted I get a 404 error message. Now I took MIGR-74675 and searched on Lenovo’s support site and got the following link. Now I don’t know if the link you had detailed specs on the power supply.http://support.lenovo.com/en_US/detail.page?LegacyDocID=MIGR-74675#pow
I don’t know what’s out there graphic wise but if you are saying that the minimum is 300 Watts and you have 280 Watts, you might not be able to run the card. Now, what I would do is see if there a power supply that has a larger capacity (Greater than 300 watts) that can fit your machine. Now I thought well, how much electrical power does the CPU need to run? I looked up the specs on your i5-650 processor and it lists the TDP (Thermal Design Power). According to Wikipedia it’s the amount of heat that is generated by the CPU that the cooling system must be able to dissipate. For your CPU it’s 75 watts of heat. I don’t know the equation (if one exists) to translate heat power into electrical power. There is an equation on the Wikipedia article for power. What I am getting at it would be great to know how much power is being used by every subsystem and add it up. Now a thought just came to me, you can get an inline AC Power meter from Amazon, link below:http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU
To monitor how much power your machine uses. I would get the specs of the graphic card that you are interested in and to see if the card can be added to your system based on how much power is left over.
I don’t know if this helps you. You can sell your system and build your own system with a power supply that can handle power wise, the graphic card that you want.