I have an annoying IRQ share between the motherboard on-board sound and the AGP slot which holds my Radeon 9700 pro.

Both use IRQ 16, and although there are no conflicts shown in the device manager, I'm pretty sure I'm getting crashes as a consequence. (Mainly during gaming).

XP pro won't let me change the IRQ assignments for either of these devices (area is greyed out on the properties box). The motherboard BIOS will let me assign IRQs for my PCI slots but not for the onboard sound or AGP.

Help! (Before I waste money on a PCI sound card, and disable the onboard sound)

(
Win XP Pro SP1, dx9, all latest drivers for everything, temps are good.
)

I dont think the problem is with the irq's, most newish boards these days share the irq's with no problems. if you want to confirm it just disable the onboard sound and play the game to see if it crashes.
Most crashes are down to the software now, irq's are not as critical of sharing as they used to be.
Check your bios settings for graphics related items, reduce the agp speed down a bit then test your game.

Just a few ideas, Good Luck

Also, is it any specific game or all games? What is it you play?

It doesn't seem to crash with the onboard sound disabled, although I have only tested this for about an hour.

Seems to crash when some sort of noisy visual effect is taking place. Games include Medal of Honour, Freelancer, Morrowind, and my newest and most irriating one: call of duty. It doesn't crash with everything though: bf1942 is fine, and a couple of others are ok too.

I've done clean installs and pretty much everything software wise I can think of.