Your computer seems to have enough power to run Ubuntu, either 14.04 or 15.04 will run fine but you may experience some "slowness" issues when trying to do the work, commonly based on several things, which may include, but not only limited to:
- RAM: Yes, it matters.
- Microprocessor speed: Yes, it does also matter.
- Your Graphic Cards (GPU): Have you considered that your computer can be fast enough to process everything, but not too fast in order to "draw" everything on screen? This is usually solved by checking the proper "Additional Drivers" installation and/or setting up the proper Graphics Resolution (Please have in mind that some GPU's won't be able to "draw" fast on certain screen resolutions, and reducing the screen size will help a lot).
- Your Hard Disk Drive speed: Which is -nowadays- the cause of the majority of the slowness in modern equipment. Remember that the delays on your hard disk drive read/write rates will result in a slower performance.
- Among other drivers/hardware based issues...
But, if you wish to have the benefits of working with Linux and more specifically Ubuntu, in a low resources computer, please have in consideration the next list of advices:
- Please consider using the 32 bits version of the regular Ubuntu LTS
- Using Ubuntu "out of the box" may result in a nightmare if your computer is not ready to perform Graphical Accelerated Tasks, such as 3D rendering, smoothing in the on-screen effects, overlay, etc. So, the usage of a different Desktop Environment or Window Manager will represent a major benefit to your computer's OS. Personally I use the i3WM in a low resources computer such as a Compaq Mini CQ10 and an Acer Aspire One ZG5. But you can also use the KUbuntu Desktop (KDE) in which against all odds, you'll have a faster experience when working on the majority of the applications and/or browsing your files and the web.
- Remember there are lots of different desktop environments which will represent a major benefit for your experience, but don't forget to adjust your screen resolution in order to lease the pain on your graphics processing unit (GPU) thus releasing resources which can be used for a different purpose on your system.
That's where I would start and I suggest you to try these advices and let us know about your experiences on this.
Good luck!