Yes the cards mentioned are faster but its all relative.
Going from a PCIe Nvidia 6200tc to a HD4670 will be a huge differance, the cards easilly handles the resolution in most of the newest games and wont tax the PSU.
Thats my recmendation anyway get a HD4670, it will fit in with what you have and give you a vast improvement in performance.
Oh and if you have 1GB of ram or less i would look to get 2
Mactronix