According to the Processor Electrical Specifications
site (linked to from SPCR's excellent Web Links
page), the max. power consumption on a P3 733 is 19.1W. I have one system that has an AX-7 on a 29W P3 1Ghz. I have it configured so that most of the time the CPU fan (80mm Panaflo L) doesn't even run, giving an idle temp of about 29C. When the CPU gets hotter, the fan kicks on at minimal speed (20% set in SpeedFan), and the max. temp is 42C. Point being, I think AX-7s with 5V Panaflos or 12dB papsts would be cooling-power overkill for your system.
Now, the AX-7 is a great cooler, and if you buy them you'll be able to use them on other Socket 370s or Athlon XPs in the future. But you will need to put fans on top of them, and more fans equals more noise. But you may be perfectly comfortable with that noise level.
Here's two other alternatives you can consider:
1) Get two Zalman CNPS6000AlCu heatsinks. With the amount of airflow you have in your case (from the CPU-side fan on the Antec and the Panaflo intake), you probably won't need any additional fans. This will cost about the same as getting AX-7s. It will be a little cramped, but not much more than with the AX-7s plus the 80x25mm fans.
2) Find a way to stick 5V 80mm Panaflos on top of your current heatsinks. This method will almost certainly provide enough cooling for you, be respectably quiet, and, last but not least, be cheap. The only trick is figuring out how to attach the fans. One way to do this is double-sided foam tape. A more professional way is to use plastic cable ties passed through the mounting holes of the fan and holes in your heatsink. "What holes in my heatsink?" you ask. The ones you're going to make with a drill, of course. And the easy pre-fab route is to buy an 80->60mm fan converter, which may or may not work for you if you're tight on space.
Ultimately, I'd say if you plan on getting an Athlon XP sometime down the line, get the AX-7 with 5V Panaflos. If you want the quietest solution possible for your present setup, get the Zalmans. And if you just want something quieter than what you have without spending lots of money, rig up some 5V Panaflos on your present heatsinks.