Thanks for your interest with the Synology RackStation, if you're looking for using the RackStation within a VMware environment, I would highly suggest using the RS810+ as this particular model is Vmware certified and you will be able to gain technical support for using this RackStation for your Vmware needs. If you're looking at using one of our other models, you may elect to do so, as they are all managed by the same OS, the DiskStation Manager - which means, that the management experience is the same, and the functionality, and abilities of the RackStation will be similar with the exception of performance.
If you want to compare the RS810+ with the RS409, the largest factor is performance, followed by scalability. You may wish to look here for the Block-level iSCSI performance of the RackStation
RS409: 54.69MB/Sec Writing; 64.82MB/Sec Reading
RS810+: 98.9MB/Sec Writing; 97.27MB/Sec Reading
Note the above performance numbers are using RAID-5 - you may elect to use RAID-0 if you're looking for greater performance, but a RAID-0 environment offers no redundancy - therefore a disk failure will result in failure of the entire volume.
With regards to scalability, the RS810+ can be paired with its companion system, the RX410, where you will have the ability of creating an 8-bay storage system, useful as your storage needs increase, you can expand the storage capability of the RS810+ without the need of migrating to another system, or using two RackStations.
To address some of the concerns that Helvick has brought up, the CS407 which was referenced is part of our 2007 product line, and was reviewed with an early version of the DSM 2.0. Today's RS810+ contains a lot more performance, and with the latest DSM 2.3 OS, provides more functionality.
The RackStation supports the ability of hot-swapping HDDs, where you may remove a failed HDD and replace it with another one and repair the volume if it is a redundant volume. Email Notification, visual, audio alerts of a failed drive is available in the current DSM 2.3 OS.
If you're concerned about having high power availability, you may elect to use the RS810RP+, which features two hot-swappable redundant power supplies, which will allow for the RackStation to be operational in case power to one module has failed.
Also, iSCSI SPC3-PR is available in DSM 3.0, which is currently being evaluated as a public beta. Please visit synology.com for further information about the DSM 3.0 Beta.
If you have any further questions, I would suggest that you contact our pre-sales team, and one of our product specialists will be happy to assist you. Please visit synology.com and click on contact us on the lower right, and you may use the Product Question form for contacting our product specialists.
Have a good day