Probably best to leave "Disk Cache" disabled ... the controller uses a battery-backed cache, so if the system loses power, the unwritten data is preserved in the cache.  HDD/disk cache does NOT have that protection, so if power is lost to the server, the unwritten data in the disk cache is lost, causing corruption.  This should only be used if you have taken extraordinary measures to ensure the system never loses power (or the data integrity is not critical). I can only guess the "Set Link Speed to G3" might actually be 3G, meaning it restricts throughput protocols to 3Gbps (SATA II/SAS I) instead of the 6Gbps (SATA III/SAS II) that it is capable of.  If you are using the proper disks (SAS or enterprise SATA II/III), there shouldn't be any reason to enable this, as it is probably a compatibility setting for SATA I/II drives whose speeds can't be negotiated properly with the controller, but if you use decent drives, that shouldn't be an issue.
Just received a PowerEdge T320 with Perc H710 controller. There are a couple of configuration options not documented in the manual, nor online help. Can't find anything on the web as well. It's about "Set Link Speed to G3" and "Disk Cache". What do they do? And how do I configure them, taking into account the server will be running Windows server 2008 R2? Simon
Perc H710 options