Don't listen to this guy. a 5450 should do benchmark fine. I had a 3870 radeon and was getting scores of 2700-2800. I upgraded because I wanted even better, but even with that score according to SE, was right in the middle.
A 3870 is a better card than a 5450.
First number = Generation
Second number = Major model number.
Third number = Minor model number.
The second number is the most important, followed by the third. Anything that is X5XX or lower is crap. X6XX is slightly less crap. X7XX is good. X8XX is high end. X9XX is top of the line. The first number is just the generation.