I am having a problem finding the dell dimension 300. I come up with the dimension 3000. So If it is the 3000 then no the 5450 will not work on that MOBO if this is your MOBO.http://cedcomputerservice.com/store/catalog/images/DELL%20DIMENSION%203000%20MOTHERBOARD%20N6381.jpg
If it isn't see if you have a PCIe x 16 slot on your MOBO. Because that MOBO does not have one. Get back to us and let us know for sure.