You're being deliberately obtuse because you were advice-abused as a forum newbie, right?
You realize the reason Derek said that is because he's testing cards with 48 pixel shaders and 50GB/s of bandwidth, a far cry from a piddling 5600 and a semi-piddling 9800. Quite explicit in that quote is that you buy a faster card to push it to its limits, and you can do that by increasing the resolution, AA, AF, or in-game effects. A 9800 will whoop a 5600 on all of those counts, in both performance AND quality.
I specifically said a P4 3GHZ b/c that's what DP has, so the benches Matthias and I linked will show him almost exactly what he'll gain. (Also, in its day, a 9800P could scale all the way to a P4 3GHz; meaning, you'd want a P4 3GHz to extract max performance from a 9800P.) Believe what you want, but know that a 9800P or 6600GT will be a significant improvement over a regular (not even Ultra) 5600 on a 3GHz system. DP should also be able to sell whatever used AGP card he buys now later this year, so his total outlay won't even be $100 for a year of smoother, better-looking gaming.
Thumbs down to your condescending tone, bad advice, and inability to acknowledge or even recognise good advice.