Installing Lion?

Since you need Mac OS X 10.6 Snow Leopard to install Mac OS X 10.7 Lion, as you need the Mac App Store (MAS) to download Mac OS X 10.7 Lion, you first need to buy the 29 USD Mac OS X 10.6 Snow Leopard Upgrade DVD from the Apple Online Store.
You can either perform an upgrade install with Mac OS X 10.6 Snow Leopard or a clean install.

To create a Clean Install (formerly known as Erase & Install) of Mac OS X 10.6 Snow Leopard (the 29 USD Upgrade DVD is a fully working retail version of Mac OS X and does not need a prior installation of Mac OS X on the Mac), follow one of the following guides:

• Short version: Clean Install of OS X 10.6 Snow Leopard
• Long version: Mac OS X v10.6 Snow Leopard: How to Erase and Install

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The issue you would have, is not having enough RAM, as Mac OS X 10.7 Lion works just okay with 2 GB RAM but really needs 4 GB to work fluently.
Maybe upgrading to Mac OS X 10.6 Snow Leopard is enough for you?