I'm using it on a private LAN, so I won't need SSL offload.
AES is an algorithm, not an application. (For example, ReadyBoost uses AES encryption on all the data that it caches for privacy reasons).
SSL and IPSec are different technologies, although both might benefit from AES-NI somewhat. Between the two I lean towards IPSec, as SSL has been superseded by TLS. No-one should be using SSL for anything.
The AES-NI will be to accelerate the full disk encryption by a factor of between 3x to 10x, thus making it fast enough to retrieve/serve spinning platters with data. Otherwise it needs to wait another revolution (or more)*, which cripples performance.
* This would require a peak encryption/decryption performance of around 750 MB/sec (for 4 drives), even if the sustained throughput is a fraction of this.
A good example to test this, is to disable DMA mode and start copying files around, with the full disk encryption enabled. The sheer quantity of interrupts going on will grind the server to a halt, even if CPU usage is 'nearly 0%' at the time. (Although in PIO Mode 4 it should hit 100% load on at least one core/thread).
Currently most cascading ciphers will give me between 40MB/sec and 60MB/sec, using 100% of both of my CPU cores (Core 2 Duo E6600, without AES-NI).
'Unfortunately' the CPU needs to be able to serve other tasks, such as running an Operating System. (The encryption uses a lot of CPU resources that generally don't show up in Task Manager, the interrupts it causes might however).
One of my objectives is: 4 HDD's in RAID-0/1 or RAID-0 while keeping CPU usage low enough for the chip to throttle into low/silent mode and giving 100 to 150 MB/sec 'sustained throughput' (locally) at < 20% utilisation and far beyond that at 100% load on all cores. (And of course being able to max out a Gigabit Ethernet link at very low CPU load).
^ This (above) would require a 9.216 GHz Core 2 Duo (to maintain < 20% CPU usage). Thankfully there are processors with AES-NI that lower that requirement into 'realistic/current' technology levels.
So long as I get an acceleration of a factor of 2.76 times, or greater, then a 3.33 GHz Core i5-660 (for example) should meet the above requirement (of < 20% CPU usage for 100MB/sec to 150MB/sec full disk encryption).
Both of the above two cases are assuming a 128 MB/sec sustained to/from the media sequentially. (4 x 250GB HDD's, assuming 32 MB/sec per HDD in RAID-0). It is likely, and highly desirable, if it can handle even more to future proof it somewhat (as 250GB HDD's are not really all that fast by today's standards, even four of them).
Updated: I edited some of the numbers/factors above as they were about 20% off!