Here's the VMware KB article concerning fault tolerance:
Hardware-wise VMware explicitly names only CPUs on which FT is possible. But then they state that HW vendors certify their servers for FT support. If you are not going to build your own box, but purchase a server from some vendor (good idea IMHO if your environment requires fault tolerance), then religiously adhere to VMware's HCL. If you really, really want to have something else running your FT machines, then e-mail VMware (and/or the HW vendor) and obtain from them a statement that they will support your configuration. You do not want to have a mission critical (why else bother with FT) system running unsupported configuration, believe me.
Getting support from vendors isn't about logic, it's about them committing themselves to help you solve problems you may encounter, provided you run something they promised to support (via HCL). Something not being on HCL may be caused by the configuration not having been tested yet (or the vendor deciding they are not going to spend resources getting this config certified) or because some problems have been detected during tests. Whichever is the case, you are out in the cold if you run unsupported configuration. Support organisation's reply will vary from "please, replicate the problem on supported hardware" to best effort support with no promise of anything whatsoever. Caveat emptor.