The GTS 450 is better than the GT 430. But what is even better is the AMD/ATI HD 6770 or less like the 6670 would work as long as you don't have a slim line tower and you can take normal cards.
Your rig is not going to be able to play on med to high settings. It will how ever play on low settings with low resolution and low FPS. If you up grade then still low settings but better resoluting and better fps.
So here are the three cards two of what I was talking about and one that you had asked about. Also the hierarchy of the GPU's as well. I hope this helps and good luck to you.
you defiantly need a decent gfx card and also a cpu cooler which will enable you to bump your cpu speed to a minimum of 2.4 ghz... if you dont bump it , then it wont matter what gfx you get because the cpu wont be fast enough to give smooth gameplay...
yeah mate that is an old machine there, with budget parts. I'd say GT430 will be enough to run the game on lowest settings, but your CPU will leave you hanging. Follow the advice hexit gave, that might give you enough juice to get decent fps.