RE: XPS 8900 freezing
As far as I know, if you're running from a SSD, you'll need to be in AHCI, not RAID.
There's a larger issue with the USB ports in some of the 8900s. Some users are reporting that they'll only get functionality out of one USB port from a panel; ie initially both are working, but shortly after plugging something into one port (say, on the top), then the companion port stops working. I haven't experienced this personally, and wonder if updating the chipset and USB 3.0 drivers doesn't help.
Those little USB wi-fi keyboard/mouse setups are junk. I can't stand them. They burn out often enough (once a year for me, on average), and I've found their performance totally unreliable. At least get one with a beefier wi-fi adapter.
Also, if you really wanted to drop that HDD into the new machine, you should be able to load it up with any necessary drivers. Just remember that if you're pulling them from the Dell website, you need to make sure you're downloading the correct drivers for Win7.