Any new mac you get should be capable of running Windows XP just fine. I suggest using a Bootcamp partition on your iMac.
This article says there's a problem with Boot Camp and XP service pack 3, but it has directions on how to get by the error.Bootcamp on the 2011 won't run Windows XP, a VM will though.
Hmm, I don't think that's the case.Windows XP doesn't support the modern hardware (Sandy Bridge) in the new iMacs. So you can't run it in Boot Camp.
You can run XP under VMware Fusion or Parallels, since they create a virtual environment which hides the details of the hardware. However, the performance of a virtualized XP system may not meet gaming expectations.
The best answer is to use Windows 7