Man, I played Doom 3 on my old lowly Duron 900 (overclocked to 957, but had to raise it to over 1GHz just to be barley playable when beating that hell guardian) and an overclocked GF3.
Anyway the parts gkay09 listed would be sufficient. I'd invest in a better motherboard though so that you can upgrade the CPU and maybe add another 4670 for crossfire latter (which also means investing in at least a decent 500W PSU). Also, if you invest in a good PSU you can always carry it over to your next build. I don't know about that 740G motherboard though as I don't know if it supports the 7750 out of the box. For an AMD motherboard I would recommend
ASRock A780GMH/128M $71.99http://www.newegg.com/Product/Product.aspx?Item=N82E16813157154
You could probably play Doom 3 with it's integrated video
. Also, the ACC is useful if you are going to overclock a 7750/7850, but if you go with an older Athlon 5400 BE or something that's a moot point
. If you would be interested in crossfire in the future then you may want to consider this board
ASRock A780GXE/128M $79.99http://www.newegg.com/Product/Product.aspx?Item=N82E16813157139