Which works best for me?

I want to improve my graphics by adding a new graphics card.
my budget is 3500.
amd or nvidia????
shortlisted:-
PowerColor AMD/ATI Radeon HD5450 2 GB DDR3
HIS AMD/ATI Radeon HD 6450 2 GB DDR3 Graphics Card
is any of the two is better or suggest me a good graphics card
my pc configuration is:
Motherboard-ASUS P5KPL AM/PS
Processor-Intel Dual Core 2.6GHz
Ram-3GB DDR2
Intel (R) G33/G31 Express Chipset
Intel (R) GMA 3100
Total Graphics Memory-256 MB