Sorry to bust your bubble, but a 8400 GS has only about 1/3 of the power of a 7900 GS. For those generations of cards, the
second number is indicating the performance level of the cards: 2,3,4,5=not meant for gaming 6=budget gaming 8,9 = high performance.
Your P4 2.8 GHz equally has about 1/3 of the power of the "minimum" required CPUs.
RAM will be the least of your worries
I would suggest to not invest any time/money in your old machine, unless you can get ciV at least to start up and run at more than a crawl. Than you might want to think about optimizing your performance ...