umm, if I were you, I'd stick with NviDia, but thats my opinion...
Fanboism? just kidding.
Why stick with nVidia? I think i would rather choose the better performance for my money.
Though i like ATI slightly better (like .1%), because of the Catalyst, the nVidia control panel is kind of confusing. But thats the only thing. Other than that, i check the cards in my budget, and get the one that performs better.
i think the old CRT are 1024 x 768.
If you plan to play at that resolution, you shouldnt need to turn on AA, only AF, i think.
So i dont think you will need that much of an high end card.
maybe the 8800GS or the 9600 GT/GSO.
As for ATI, im not really sure, i never researched them and saw any benchmarks because back then, the 2 cards i could buy is either the ATI HD3870 or the nVidia 8800GT, and the 8800GT performed better so i went with that.
But i think the 3850/3870 from ATI should be good.
I dont think i saw any benchmarks for the new 4xxx series and the 9800GTX/2xx series, i dont know how they perform and assume what others say about the cards are correct.
I think the 3450 should be good for your system, though i dont know how long it will last, you might have to change graphics card really quick if you plan to max out a lot of games within the next year.