I wouldn't want to trust a RAID to any other than the three listed (HiP, Sonnet and Firmtek) with Firmtek being my choice due to the "native" support that the controller enjoys and the commitment to support and performance that they have shown through the years.
Sonnet is a good choice, primarily due to the fact that they use Firmtek controllers on many of their cards.
Sonnet has shown some good performing cards now and then.
High Point has some performance hardware RAID cards, is historically a performance oriented Mac supporting supplier, but software and support issues may be incurred from all accounts that I hjave seen through the years
Still, a much better choice that going down the Sil Images route.
Other options, in OS X, would only be cards that use specific Silicon Images chipsets and drivers. With these cards, chipset is limited to specific versions, and OS support is often limited to a specific update of the OS; 10.4.7 only as opposed to 10.4.1 through 10.4.11 (example only).
If you wish to investigate, the relatively common 3124 chipset and OS supported are listed in the following:
http://www.siliconimage.com/support/searchresults.aspx?pid=27&cat=3
Other possibilities might exist. Use teh pulldown menu and do some searching:
http://www.siliconimage.com/support/index.aspx
Of course, you might consider a Linux server, then all of this discussion is mute and changes with that which is supported in Linux.....