It would be best to buy the 5450 card. But a few things first.
1) what is your budget
2) what Res do run and what games do you play?
3) what power supply do you have?
4) I would upgrade my memory to at least 4gb
Reason I asked, you could get a sapphire 5670 card for $89 with a rebate.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102869
But if the 5450 is all you need then make sure you get a 1GB DDR 3 over the DDR2 cards.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161322
Also checkout this video - Starcraft II running on a 5450 1GB DDR3 Cardhttp://www.youtube.com/watch?v=OJD9RVj_SZg