I currently have a mac book, about five years old. It works great except for the fact it will no longer (happily) run two programs at once (safari + excel or itunes = slower than a snail).
Should I just buy more memory for it (that would help it, right?), or get a new mac book, and use this one as a back-up?
Before you spend money, try Chrome as your browser. Something happened to me about six months ago where all of a sudden my MBP (4 yrs old) was dragging ass and it did it whether I used Safari or FF, even after clearing cache/cookies and all that other standard tech support BS. Started using Chrome instead and all my performance issued disappeared. (Strangely, I had this exact same experience at work on my winbox)
Also probably wouldn't hurt to use mackeeper or some other cleaner utility... but try Chrome first and you may solve all your issues.
That said, memory is important. I buy for my company as well as myself. I generally buy 1 or two processor speeds below bleeding edge to save money there, but max out on memory (for macs I do that through the authorized retailer since they can do it cheaper than ordering the mac with the memory... they can usually do an HDD upgrade too for cheap if you do it before you take delivery).