Upgrade T9400->QX9300 = BSOD

Well, yes. Originally, I wanted to put a Q9100 into my E6400, but when I put that into my computer, it seemed that my BIOS didn't really like the quad-core. It only actually booted randomly, perhaps 1 in 10 attempts, and when it did, it only used 2 cores.
By that time, I had sold my T9600, so I decided to just get an X9100. When I put it in, I removed the dell thermal pads and applied Arctic Silver (and did a copper mod to the GPU), and I undervolted my X9100. So, everything is running pretty much optimized. However, it does generate quite a bit more heat, mostly because the E6400 isn't designed to handle the 44W TDP.

My fan doesn't really kick in to high speed until about 55-60C for the CPU, which means it's pretty quiet most of the time. It's almost always at it's lowest speed, mostly because my northbridge/ACPI temps have gone up since I did my GPU copper mod - about 50C at normal use (I suspect it has something to do with the thickness of the copper affecting the angle of the heatsink assembly - see the link in my sig).
To give you an idea of my temps, at normal use, let's say, skype running, some music playing, a bunch of tabs in google chrome, and word, my CPU is at around 43C. If I start playing youtube HD, the CPU may go up to 55C. Playing a game like GTA IV takes it up to about 70C, Rendering in Adobe Premiere Pro brings it up to about 75C. Running a stress test in ORTHOS (which simulates more stress than possible in real world applications) pushes the temps to 85C, but I've never really gotten them that high in real world use.

The bottom line is, that you should probably go with the T9900, if you really need every extra mhz of clockspeed. Honestly, I barely feel a difference between the 2.8ghz T9600 and the 3.06 X9100. But I got my X9100 before the T9900 existed, so...