The video memory of 7750 is gddr5 and it is accessed by the card directly unlike accessing your system memory when that vram gets used up.
So,be your system memory a ddr2 or ddr3,you'll have no problems in using the HD 7750.
You may try looking at the label of your PSU for the wattage it delivers.
Normally a 300W PSU is enough to deliver 75W to the pcie express slot and the card requires a little less than that.
(Note:The label attached on the PSU shows some wattage in several columns(+3 -3V,+5 -5V,+12 -12V).If you see the wattage in +12V column being around 130W or above,you're good to go.The combined wattage from all the columns describes your PSU wattage such as the 300W stated above.)