Well basically it depends on what features you are using.
I'm not 100% sure of IIS 5.1 (the one XP has), but if I remember right you are limited to one website or so and only a number of concurrent connections, but for your scenario that shouldn't be a problem. But maybe someone else knows more.
With regard to SQL Server, just check the
feature comparison chart and ensure you are not using one of the features that is only available on the "big" one.
As an alternative you could always install eval versions of the big SQL Server (you were saying the laptops are high spec) for testing, or even create the test databases on your main database server (if it's not a production but rather a development server).
HTH