I don't think software is going to help you here. You're right at the minimum required RAM (or below, depending on who you ask), and below the minimum CPU speed listed on the Ubisoft Store Page (I've added the specs below my answer).
The answer is in the hardware, upgrade!
Minimum Requirements from the Ubisoft Store Page
- Supported OS: Windows XP/Windows Vista/Windows 7
- Processor: Dual core processor 2.6 GHz Intel Pentium D or AMD Athlon 64 X2 3800+ (Intel Core 2 Duo 2.2 GHz or AMD Athlon 64 X2 4400+ or better recommended)
- RAM: 1 GB Windows XP/2 GB Windows Vista and Windows 7
- Video Card: 256 MB DirectX 9.0 or higher compliant card with Shader Model 3.0 or higher (see supported list)
- Sound Card: DirectX-compatible sound card or motherboard sound capability (5.1 sound card recommended
- DirectX Version: DirectX 9.0 or 10.0 libraries
- Hard Drive Space: 8 GB
- Peripherals: Keyboard, mouse, optional controller (Xbox 360 Controller for Windows recommended)