You're comparing a card that has been released a year ago and a card that is over six years old. However for gaming, a 5450 is too weak.The cheapest gaming card that I'll look for is the Hd 4670/ 4650 or th 5570/ 5550 GDDR5.
It is weaker than a 6800. Are you sure you want to invest in a card that is weaker than your previous one? HD 5670 is a very good card for older systems.