^ The GTS450 GDDR3 is actually isn't that far behind. In fact, it's still ahead of the HD 6670 GDDR5. I'm saying this from experience, cause I own a GDDR3 450, and it works pretty well, almost at the same levels as a GDDR5 card.
Th memory type won't affect it too much. I'd say about a 5-7% performance drop from the regular GDDR5.
But you could look into more powerful cards at that price range (assuming it is around $100). You could take a look at the HD 7750 which is faster than both these cards.