Mac OS X 10.7 requires at least 2 GB of memory....

It says here: http://www.apple.com/macosx/specs.html that you need 2 GB to install Lion.

General requirements

• Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
• 2GB of memory
• OS X v10.6.6 or later (v10.6.8 recommended)
• 7GB of available space
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

2 GB of memory means 2 GB of RAM, not space remaining on your hard drive. How much RAM do you have on your Mac? You can find out by clicking the Apple in the menu bar on the top of your screen, clicking "About this Mac" and it will show how much you have under the "Memory" section.

I have attached a screenshot for you:

I may be wrong, but I don't think that Apple was selling any Mac meeting other hardware requirements for Lion with less than 2 GB of RAM.

Are all your Mac's meeting other hardware requirements? A quote from Apple: "Your Mac must have an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor to run Lion. Find out if your current Mac has one of these processors by clicking the Apple icon at the top left of your screen, then choosing About This Mac."

Please post the model identifier of your Mac so we can point you in the direction of the correct RAM. Also, keep in mind that 2 GB is the minimum required; however, you would be much happier with more than that - 4 GB are highly recommended. That is why we need to know your model so we can figure out what the supported maximum of RAM is in yours.

You need to install more RAM in the computer. This is not hard drive space.

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You can upgrade your RAM if you want to upgrade to Lion. What kind of Mac do you have?

Yep. You will need an extra GB of RAM to get Lion. You can usually do this yourself or have Apple do it for you. Either way, you will need to buy more RAM.