You should ALWAYS use the latest version, for security reasons.
System requirements
Windows
- 2.33GHz or faster x86-compatible processor, or Intel® Atom™ 1.6GHz or faster processor for netbooks
- Microsoft® Windows® XP (32 bit), Windows Server 2008 (32 bit), Windows Vista® (32 bit), Windows 7 (32 bit and 64 bit), Windows 8 (32 bit and 64 bit), or Windows Server 2012 (64 bit)
- Internet Explorer 7.0 or later, Mozilla Firefox 17 or later, Google Chrome, or Opera 11
- 512MB of RAM (1GB of RAM recommended for netbooks); 128MB of graphics memory
Mac OS
- Intel Coreâ„¢ Duo 1.83GHz or faster processor
- Mac OS X v10.6, v10.7, v10.8, or v10.9
- Safari 5.0 or later, Mozilla Firefox 17, Google Chrome, or Opera 11
- 512MB of RAM; 128MB of graphics memory
Note that a Corei3 or Core15 is "hyperthreaded" and functions like a quad core processor. Additionally, dual core processors have a "net speed" equal to the processor speed X the number of cores, so a 1.3 gHz dual core is really 2.6gHz.