Dell Poweredge SAS issue

For Dell, "256MB Cache" means 256MB of battery-backed cache on the RAID card. This is the cache used by the card for I/O operations. It caches writes in this storage to re-order them for more efficient usage of your disk. The battery is there in case of power loss, the battery supplies voltage to the cache-RAM. I believe it lasts up to 2 days.

"512MB NV Cache" means non-volatile. I don't know exactly how Dell does it, but I strongly suspect it has a normal RAM-based cache. A super-capacitor on the card has enough juice to commit the cache to flash-memory in case of sudden power loss. This will survive a power-outage for a lot longer than a battery-backed cache.

1GB NV Cache just has more cache.

The amount of data you're slinging around isn't that large to multiple internal connectors is not that important.

If you trust your power environment very well, a battery-backed cache will save you some money versus the non-volatile cache option.

http://support.dell.com/support/edocs/storage/RAID/PERC6/en/UG/HTML/index.htm

Couldn't locate drive mixing in my brief search of the manual, but I am fairly certain you can mix drive types in the server/controller (SAS/SATA/SSD). You cannot mix drive types in the VDs (can't have a RAID 1 with a SAS and a SATA drive). It's been a few years , but I've used a MD 3000 before which was pre-PERC 6 and you could mix drive types in that enclosure (SAS/SATA) so I assume you can do the same with a later PERC. It shouldn't matter what your first VD drive type is either. I would typically make a RAID 1 SAS for the OS, then whatever else is required afterwards for the server.