The Windows Phones like 520, 630, 635, 620 etc. are with 512 MB RAM and Lumia 520 is one of the most sold Windows phone in India too. Windows phones work nice even with just 512Mb of RAM. Here are some cool games for entry level Windows Phones with less memory.
Subway Surfers a popular game on all platforms is now (From July) supports Windows phones with 512 MB RAM. It is official.
You can get the game from Windows Phone Store
The Nokia Conversations posted a list of cool games which you can play on the phones with just 512 MB RAM. These games are paid ones.
Halo: Spartan Asslt. – This game ($4.99) hit Windows phones recently. Halo: Spartan Assault brings the excitement of Halo combat to touch-based devices for the very first time. Battle your way through 30 action-packed missions against the Covenant as you explore the origin of the Spartan Ops program and Halo 4's Spartan Commander, Sarah Palmer.
You can check other games CarneyVale: Showtime, Harbor Master, Jet Car Stunts WP, Farm Frenzy 2 on this pot on Nokia Conversations
If you are looking for Free Windows Phone games check these features games on Windows phone store or the Top free games for Windows phones here