I think with them specs your best not to use PS but you could try elements which is a smaller version of PS having just "elements" of the PS. Or you could try PSP (paint shop pro) I have version 8 and it dont use as much resource as PS. With regards to selecting just the parts that are on a certain layer you could try this but remember I am using PS so might be diferent on what your using, I would;
Hold CTRL and click on the thumbnail of the layer and you should see the marching ants around the parts of that layer that contain pixels ie; it's now selected.
Also in PS there is the option to lock the transparencey which means that only the pixels will be effected and not the whole layer.
Not sure if you've got the option to save a selection or create a path but that is also a option in PS.
Hope this helps.
The pic's you have done so far are great! Well done.
Thanks to TA + J 4 the big up!!!