Yah that vidio card should be fine,the cpu should be fine as well but if you want to upgrade i would go with the ahtlon 2500+ and overclock it to a 3000+ it will lower the life time a bit but u can prob pick one up for around 60-70$ maybe lower if on sale.

To install your new vidio card........

-Shut down your PC
-Unplug the power cable and all other connections from the case
-Open the case
-Ground yourself of static electricty by touching something metal
-Find the vidio card and unscrew the screw holding it to the case
-Remove the vidio card from the motherboard
-Install the new vidio card into the same slott
-Close the case
-Plug everything back into the case
-Start up your PC
-If your card didnot come with a CD with the latest drivers go to the website and find your vidio card and there driver section
-Download and install the drivers and restart your PC

There you go u have a new vidio card and are ready to play some doom3

Good luck and post if u need further help.