In order of impact, it would depend on what you have in there now. The stock fan on an AMD retail chip is significantly louder than an 80mm fan on an after market heat sink for an OEM chip, for example.
Why don't you tell me the specific things we're working with here. Are you using the retail heatsink with your P4? See if you can figure out which model of PSU you're using as well.
That Radeon probably has a tiny little fan running at very high RPM's. You aren't hearing it right now over the other two fans and the CPU cooler, but it will drive you nuts when you get them quiet. Zalman makes a passive heat pump that will make zero noise for that cooler.
An L1A at 21dB is pretty hard to hear. Additionally, if you're not overclocking, you may not need that much cooling. Download Motherboard Monitor and try to set it up. It will let you use the system's temp sensors to monitor your CPU and case temps from within Windows. Pull the two fans and see what the temp difference is. So long as you stay under 60 degrees for a full load on the CPU, you'll never have a problem with cooling. Above 60 is when I start getting uncomfortable.
A good enough heatsink for your CPU to allow for a low flow fan like the L1A will run you $35 or so. Pair it with a Panaflo and you're golden. Pulling the two case fans in favor of a Panaflo, or better yet nothing, will make a difference as well for not much money. The Zalman cooler for your vid card is in the neighborhood of $12 I think. The most expensive upgrade would be the PSU. A True350 from Antec is around $50 I think.
