Need advice on Lap tops.

Currently, I have a HP Notebook. It's about 4 and a half years old. It is starting to really drain the battery quickly ....Or the battery is just bad. Two questions......Is it worth investing in a battery for a computer as old as mine? Second...IF I do get a new one, what is the best for simply surfing and sending e mails. I do not store a whole lot on my computer. My Wife paid about $500-$600 for this one. Is that the going rate for good or average these days?