Well lets see here. On your PSU is a sticker, on the sticker are a bunch of numbers. The number or numbers I am look for are the amps on the 12v rail. Some PSU's have two 12v rails.
You will probley only have one. If you could look and write down all the numbers that go with these for example 3.3v = 15a + 5v = 14a + 12v = 14a. If you have more than one 12v rail it will look like this 12v1 and 12v2.
Your 5450 only needs 19w on max load which is good. So the amps on the 12v rail should be and to be safe with head room 12 to 14 amps. You should be able to run that card but need to be sure first.
Get back to us with the info and we will be able to tell you good to go or not to go. Anyway good luck.