is there any computer with pci-e to play sc4 RH at good spee

hello to all
i want to ask u people a question
i am a big fan of sim city 4 (rush hour) and i have a relatively old machine
pentium D805 (2.66GHz) Asrock motherboard with ati200Express , radeonX800xt (256 mb gddr3) and 512 mb of ram
i want to upgrade my system just to play simcity 4 (yes u read right ,just in order to play SC4) it runs ok with my current system but once the population is very high than the speed is pathetic and i ultimately loose interest in plaing the game
has anyone of u a system on which he or she is playing simcity 4 with good graphics and with high frame rates with a population of 100K (my highest was 150k but than because of less speed i couldnt continue playing it)
any recommendations!
ive seen a post in which the guy has described that he has a system like e6600, 4gb of ram ,1900xtx but still he is not able to play the game smoothly.
i wonder if anyone plays the game with pci-e card falwlessly? (with agp i think its possible with ati 9800se or so but i dont want to go back to agp)
will b thankfull if anyone can answer
one thing my game also runs in software mode and i m playing other games with much much better frame rates as compared to sc4.