Two years ago bought a Dell XPS 630i with dual GTX9800s. It constantly locked up with SLI enabled - there are dozens of such complaints on this system at Dell's support forums. I replaced the power supply, HD, both GPUs, upgraded OS, reinstalled OS, disabled onboard sound and put in a sound card - all in response to various suggestions; also dicked with voltage to various components, swapped memory, etc., all to no avail.
So just for shits&grins I e-mailed Dell to complain and they responded that I had done son on the very last day of my warranty - I thought the warranty had expired after a year. So then sent a tech by for a mobo replacement which didn't fix the problem. Just got an e-mail from them that they're replacing this two-year-old system with an Alienware Aurora. Don't know what model it will be but with an Aurora, doesn't really matter. It's gonna scream.
The moral of this tale is never give up. Pretty generous of Dell as well.