If I were on a tight budget, here is what I would do:

Purchase two linksys Gig-E switches, and make sure that half of my POS terminals were running to one, and the other half to the other switch. Purchase a couple of desktop computers on ebay with enough RAM for your requirements, two hard drives each (so you can do hardware RAID) and a hardware RAID controller, and an additional NIC with dual ports. You'll want to stack the two switches, or at least trunk between them and turn on spanning tree.

On the dual port NICs on each server, run one Gig-e cable to one switch, and the other cable to the other. Create a crossover cable and run it directly between the two servers to the onboard NICs (this will be used for DRDB). Download CentOS. Set the dual port NICs in an Active/Passive Mode. Download DRBD: http://www.drbd.org/ And create a disk partition that is mirrored between the two nodes with GFS running on top of DRBD (you now have shared storage, functionally similar to a SAN). Download KVM, or some other virtualization product and run your SQL server/.Net application as a virtual machine. Given your environment, it doesn't sound like you'll need high IOPS, and so this setup should work fine. You could use the clustering software that comes with CentOS, but in the case of a physical server failure, it would probably be easier to simply spin up the instance of your virtual machine manually on the 2nd server with the KVM software.

Granted, you could make things work seemlessly, and I've done that before, but it takes a lot more work than I'm guessing you would wish to spend on this project. By approaching things the way I mentioned, you could easily have a small list of directions that your wait staff (or shift manager) could follow in the event of a physical failure. It would be easy for them, and you could ensure that your business operates with excellent continuity. Your total expenditure if for everything I mentioned should come to about $1,200 - $1,500 for the entire environment. Finally, in the event hat Murphy's Law visits your establishment, be sure to have a backup strategy.

Now, everything that I discussed isn't especially easy, but there are TONS of resources if you spend some time on Google. And, it is definitely all very doable. My background is in designing highly available infrastructures internationally with budgets ranging from mom & pop small town establishments to multimillion dollar infrastructures.

Good luck, feel free to contact me at: ATheoryOfTruth "at" Gmail . Com

Cheers,
Andrew

I just noticed the bit9 comment and got excited because we're bit9 customers. I've locked down our call center and customer service reps' computers with Parity and have plans to eliminate mcafee corporate antivirus from our environment. I love the software and our employees (ones who previously enjoyed installing unapproved software) despise it. It catches just about every type of executable and can very versatily handle software installers/packages. Between that and Flexnet Adminstudio a Windows admin can very effectively manage an AD enterprise. Between costs for licenses/network bandwidth in corporate infrastructure/manageability/etc, Parity and adminstudio has been the most costeffective and administratively effective solution I've been able to find. Note: this is for windows environments only, I'm assuming here that Sunrays, linux thinclients, linux workstations, etc etc etc are not an option.

Cheers!
drew :)