I also have the Dell T3400 and its dual NVS-290 Nvidia based graphics cards support 4 monitors, as you well know. Given your situation (and mine), I simply chose to purchase a brand new "back-up" computer from the Dell Home Outlet site. An XPS-8300 which is being discontinued in favor of the newer XPS-8500 that comes with the 2nd generation of i7 core processors, but seems to be less configurable given Dell's latest penchants to cut costs. I put the NVS-300 PNY Technologies card (that I mentioned in my previous post) in the XPS-8300 and that's the avenue that I chose to supporting 6 MONITORS. I was also able to load up the XPS-8300 with Windows 7 Ultimate in the 64 bit version which allowed me to go above and well beyond 4 Gigs of RAM. I purchased 16 Gigs from Crucial for $92.00
You should leave your T3400 as-is and find a use for it. Upgrading options are limited, but you could run 3, quad cards to support up to 12 monitors (2, x16 450s and 1, x1 450).... with W7 OS, of course.