Seems like there are a few people @ Dell's forum that have the XPS 630i, and are running Q6600 and Q9300 CPUs.
I would look for Dell's official stance on whether or not the CPU is compatible with your system, but looks like it should be (XPS 630i using modified nVidia 650i mobo, according to what I read). Like CompuTronix mentioned, a BIOS flash might be needed.
dell modifies the BIOS to rebrand the machine into a dell machine, this is why you can reinstall windows onto a dell machine from a dell branded cd and not be harassed by activation or keys, it checks for the "dell" bios
other than that, the board should work the same as a regular 650i based board
i would use CPU-Z to check what chipset is on your mobo and then use that info to see if your chipset supports q6600
you may run into a proprietary heatsink/fan system which you might have to end up using or removing completely from the case, open up and see if it is a regular intel heatsink/fan or some dell one
well i have tried contacting dell in bangalore india and to be honest i dont trust them they don't seem to understand basic hardware compatabilities. and i have tried online and cannot find much info on my question. my chipset is northbridge is nvidia nforce 650i-sli (c55)
southbridge is nvidia nforce 430i (mcp51). it only uses a heatsinc no fan and it looks like an ordinary one with 4 holes that the heatsinc clips into. i cannot beleive how dificult it is to find out about the compatability of this mobo
Dell can be quite frustrating at times. I called to ask them a simple question about whether or not my dell laptop power supply was dual voltage 120/240 because I was going to travel overseas for several months. I *knew* the answer but wanted to verify... i mean this was my laptop we were talking about. I got bounced around about 6 different people before someone finally even understood what I was asking, and that person still didn't know. Needless to say I just took my chances with the specs i found from a 3rd party website. I still don't recommend it though. The first time I plugged in my laptop I was feeling a little nervous lol.