The P3 is a great chip, but its not the best for overclocking. They have those cores maxed out at about 1ghz.
I think that P3 is still noticeably faster under 3d, but a duron 700 overclocked to 1Ghz is flat out, hands down the best deal you can get. Spend the extra money on a Elsa Gloria video card (retails for about 500 dollars!)
Those two things combined will pretty much kick the cr@p out of any system out there. That Gloria card is basically a Geforce with all the features turned on combined with the best memory that money can buy.
Why spend all that money on a processor that will be out of date by this time next year? That video card will take about 2-3 years before it gets outdated, but right now there isnt anything even close. The Geforce2 ultras are about half the power as a gloria.