Basically you need an intel Mac with something newer than a core solo or core duo processor and at least 2gb of ram. They also point out that almost anything purchased since late 2006 should be fine.
We also agree with them on saying that you should upgrade to 4gb of ram if you have not already. Minimum specs as never been the same a "suggested" spec. Our favorite place to buy good quality ram guaranteed right for you mac is still Crucial. Be sure to verify the ram is easily "user installable" if you are going to purchase from the web and install yourself. Apple has been good about making sure at least this can be easily done by the end user.
One last thing to mention. Since Lion will only be available via the App Store on the computer, you will either need to be currently running Snow Leopard or you will have to upgrade to that first. Both Snow Leopard and Lion are priced at $29.
Take care and have a great day.