Umm, pardon me and with the utmost respect, I have to disagree here. That P4 is going to hold a 4850 or a 9800GTX well below their full potential. I know they don't cost a lot these days, but they are overkill for a P4. And to top it off, If all the OP does is the Nascar thing, and Photoshop. Neither are very GPU demanding, especially Photoshop.
I would suggest something in the range of a 4670 or a 9600GT depending on which camp you prefer. That is plenty, plenty of card for your use, and system I am afraid.
To the OP, you do realize how dog slow that P4 really is compared to a modern processor, right?