I am starting one POS (Point of sale) project. Targeting system is going to be written in C# .NET 2 WinForms and as main database server We are going to use MS-SQL Server. As we have a lot of POS devices in chain for one store I will love to have backend local data base system on each POS device.

Scenario are following: When main server goes down!! POS application should continue working "off-line" with local database, until connection to main server come up again.

Now I am in dilemma which local database is going to be most adoptable for me. Here is some notes for helping me point me in right direction:

1. To be Light "My POS devices art usually old and suffering with performances"
2. To be Free "I have a lot of devices and I do not wont additional cost beside main SQL serer"
3. One day Ill love to try all that port on Mono and Linux OS.

Here is what I've researched so far:

1. Simple XML "Light but I am afraid of performance, My main table of items is average of 10K records"
2. SQL-Express "I am afraid that my POS devices is poor with hardware for SQLExpress, and also hard to install on each device and configure"
3. Less known Advantage Database Server have free distribution of offline ADT system.
4. DBF with extended Library,"Respect for good old DBFs but that era is behind Me with clipper and DBFs"
5. MS Access
6. Sqlite "Mostly like for now, but I am afraid how it is going to pair with MS SQL do they have same Data types".

I know that in this SO is a lot of subjective data, but at least can someone recommended some others lite database system, or things that I shod most take attention before I choice database.