I have a Dell Inspiron Mini tower with a 300 Watt PSU. I'm upgrading my system part by part (PSU then motherboard then graphics card, case etc.) and I'm wondering if a CORSAIR CMPSU-750TX 750W ATX12V would fit in my minitower?
Yes, it will.
The TX750 is 6.3 inches deep ( the Dell psu is usually 5.5 ) and has a much larger cable bundle, so it can be a pita, especially if you have dual optical drives that will leave no room to hide those wires.
The Antec TPN750 might be a better option for you, only 5.9 inches deep and semi modular, with the current rebate it ends up costing less as well.http://www.newegg.com/Product/Product.aspx?Item=N82E16817371025