Upgrade Hard Disk Or Memory?

Should I upgrade the computer hard disk or the memory? My computer has only 60 GB storage now. Someone told me that my computer would load slow if I am store too much of data in computer. I have installed quite many program in the computer and the speed really become slower now. But is it the loading speed slow is because of small hard disk? I am not sure about it because I heard that memory does effects the speed too. Hmm..but with my limited budget, I do not think I could upgrade twp at the same time. What to do now?