It should be safe to put in a T7600 or any other Core 2 Duo chip excluding the the Santa Rosa chips/ Centrino Duo. My Alienware Superman laptop M5750/M5790 is the same chassis as your Xi1546. They're the same and I currently have a T7600 in my laptop.
As for upgrading options the T7200, T7400and T7600 all have twice as much cache as your current chip. Best value money wise would be the T7200 at 2 GHz. You wouldn't really see a big difference with the T7400 at 2.16GHz and the T7600 at 2.33 GHz in real time. But if you want the T7600 then go ahead. Here's a link to some comparisons of the different chips.http://www23.tomshardware.com/cpu_mobile.html
Hope that helps.
Oh yeah, since you do have a Xi 1546. I was looking for a spare AC adapter for my laptop but Alienware no longer sells them and was wondering if you know where to get a spare AC adapter for the Xi 1546/Alienware laptop?