So ram is the main thing I need to be looking for? Iv been told to get an external SSD drive and install light room on to that and I also have a 1tb external hard drive that I would store my photo on anyway
Here's a human analogy: You have a small desk, but too many paperwork all over the desk, as well as a typewriter, pens, books, and it will slow you down, because you're spending too much time lift up other papers to check for the paperwork you want which may be under the pile of papers.
So a low RAM like 1GB or 2GB of RAM is like a small desk to the computer. If you have too many application software open, like LR and PS at the same time, plus iTunes or whatever music player you use, and you're opening too many photos in Photoshop, plus working on complex editing. Computer gets blogged down. Computer telling you
Out of memory is really more like computer telling you "I've got no room to work on another file!"
You as a human would move some of the paperwork off the desk and put it somewhere else, in a box, in a filing cabinet, whatever, in order to free up bit of space on your small desk.
Computer do similar, that is done by
Scratch Disk, meaning computer puts files you are not working on, to the HDD, and when you want to work on them, computer swap inactive files to the disk, and bring active files from disk back to the memory. Hence the tiny noticeable delay before the file you want to work on becomes active.
So if you as a human want to work on more paperwork, what would you normally do? What would be your best option? If you suggest taking the paperwork to the table in the dining room because the table is bigger than the desk, then correct, and there is your answer...
Get more RAM for the computer to work with. The more RAM (ie: 4GB or 8GB or 16GB) you have (within your budget) the more files the computer can work with, specially with LR having to preview so many thumbnails, and Photoshop doing a lot of complex editing.
A 1TB external drive is fine for storage but is not fine for computer to work on top of it. To a computer, an external drive is like a briefcase to store and carry the paperwork, but not suitable as a workspace for you to write on your paperwork on top of a briefcase. It is good for saving your photos but computer still need more RAM to work on your photos.
When thinking about buying a new computer, RAM is likely to be the first thing most people would look for, but everyone is different, after that, worry about other specs like HDD space, CPU speed, etc., (If it was a portable things like iPad, memory stick, MP3 player, external HDD, it is mostly storage size they look for first.)