Disable power saving features. You have a second PCI slot, try using it, even if you have to rotate other cards around. You wont be able to manually configure IRQ's in Windows 7, and I'm not sure if your hardware will support trying to do it manually either. Windows 7 will automatically set IRQs, regardless of what's "best." Windows 7 doesn't take into consideration that Delta series cards need a lot of bandwidth for recording or even playback at full 24/96, and would take a lot more work in Windows to change that. That's a job for M$.
Your stuttering problem is coming from the amount of bandwidth that the graphics card is taking over the sound card. Disabling the power saving features might be able to help that, but you'd have to test it.
Also, because I'm quite well aware about computer hardware (working in a computer shop, building high end CAD workstations), you aren't going to get any benefit out of using two network ports at once. It doesn't matter if you unplug it or disable it in the BIOS, but you might see a difference in that. I'd also strongly suggest not using nVidia software tools to manage network flow, because unless it's packet filtering, you aren't going to see any benefit to it. Additionally, that software can conflict with the 2496, and be causing other problems, but we're going to deal with what's at hand first.
I would suggest disabling PhysX in the nVidia control panel unless the game you're playing actually uses it. This can also cause a problem with the Delta card because it means the graphics cards need a lot more bandwidth during gameplay.
Third party software like "Driver Sweeper" are utterly useless. You should uninstall drivers yourself, through the uninstall utility, and run a good Registry Cleaner. CCleaner or Norton Utilities. Don't leave software you don't use installed on your computer, and don't download bloatware like Driver Sweeper, it wont help you at all.
Since you've stated that XP and 7 both stutter the same, this is clearly a hardware problem, that you need to try disabling power saving features for, or swapping the PCI slot you're using.
Lastly, I'd suggest only having one sound card installed in your computer at once.
Core 2 Quad Q9400-Scythe Ninja Rev A|MSI Neo P45|WD 150GB Raptor|WD 250gb SATA-I|8GB G-Skill DDR2|WD Blue 640GB SATA-II|Logitech KB|Apple Mighty Mouse|ATI HD4850 512MB-Accelero S1|M-Audio 2x Delta 1010|Delta 66 Omni I/O|Corsair 400W|Cooler Master Elite 330
Dual Boot: Ubuntu 10.04 x64 Lucid/Windows 7 Home Premium x64
Adobe Audition Mac & PC, Logic Pro, Ardour, PT 9, Reaper x86 & x64, Ableton Live Lite
2x Delta 1010, 2x Delta 66, AP2496, FW Solo, Pulsar II, ProFire 2626, Fast Track C400