The Emu does it's mixing and fx with the onboard dsp and any irq conflict will cause a break in communications with the cpu since you have 2 (or more) devices vying for a share of the cpu's time.
Think of IRQs as highways that can support a limited amount of data. In your case, you have a high output device on your particular lane (the video card) sharing the same lane with another high output device (Emu). Audio is VERY sensitive to breaks in the data packet stream and if the buffers don't replenish quick enough, you will get the common problems of popping, clicking and other audio artifacts. Video, OTOH, just loves to take control of the lane it's in - most video card driver programmers think of every conceivable means to pack a LOT of data into the pipeline going to the cpu; and they even cheat by 'fooling' other devices into giving up their share of cpu time. As luck would have it, your Emu shares with your 7200GS, not a good combination.
Why do the other instances only get affected? The way I see it, the first instance is sending out just the right amount of data packets that fits in the spaces allowed it by the video card data stream. Adding another instance just overwhelms that stream and guess who takes control of that IRQ? You guessed it... the video card. AGP mobos had PCI Latency Tool so that IRQ latencies (not to be confused with audio latency) could be adjust so that a majority of onboard devices would 'sync' the the data highway's 'speed' and, like highway onramps, just merge perfectly with the data stream. The advent of pcie video cards (and other pcie devices) has made PCI latency adjustment impossible for now due to reasons that are too complicated for me to explain in layman's terms. Suffice to say, the door is closed to end user adjustment of PCIE bus latency.
Btw, not only Emu is affected by the need for a solo IRQ. As a matter of fact, ALL soundcards are recommended to have their own IRQ if you just want plain stable operation. Some other card's driver can do a better job with sharing; soundblasters, for ex., can share with video, but the blaster's emphasis is mostly for gaming. In most gaming benchmarks, you will see that adding a blaster improves on framerates; that means that the blaster intentionally takes 2nd seat to video, something you wouldn't want your DAW to be in. It's also one of those reasons why most DAW builders don't recommend putting that latest 8800GT into your DAW machine.
hth,
