You can only buy one with a pre-installed OS (assuming you buy from DELL's site, of course). The only options are XP, Ubuntu or Win7.
What I would suggest is to research what options there are to do the Hackintosh install, and then customize the 10v accordingly. It really depends on how many other systems you have to build out the MacOS flash drive install, and create the USB flash drive to downgrade the BIOS.
If you are still in doubt, I'd suggest buying it with XP to keep the price down vs. Win7. And probably avoid Ubuntu - not because Ubuntu is bad, but because it won't help you with your goal of making it a Hackintosh.