When you're lagging, take a look at your disk i/o lights. Adding another videocard eats away at your physical address space, but since you're running 64-bit Vista, this shouldn't be an issue. Since you've just installed a new videocard, I imagine the game has reset your /console maxfpsbk and /console maxfps settings. You may want to adjust them.

Also, running 5-6 instances of the game on a dual core 3800+ is going to be an issue for you.

I have an ASUS A8N32-SLI DELUXE w/ a 4200+. I had lots of problems getting this to work on Vista since they want to update your drivers for you. I don't know about you, but I use the nVidia raid controller and every time that bastard auto-update receives the nVidia SATA/Raid driver update, my computer no longer boots. Make sure that your system can see all 4 GB of RAM, and that you're not using the Physical Address Extensions (PAE).