its not about the first number of the graphics card, but the 2nd. you have a HD5450, which is a x4xx card, really low end. x9xx and x8xx cards (5850, 6870, 4890 etc) are considered high end usually, and are much faster and better in gaming than the x4xx card you have.
the first number simply denotes how new the card is. when comparing similar cards from different ranges, the new one is usually faster.
Eg: 54xx vs. 64xx. 64xx cards will be faster even if they are the same class.
NOTE: General guidelines here. There are a few exceptions when it comes to actual performance.
EDIT: Oh yes, the suggestion. Get a HD6850 + Corsair CX430