I would like to find a tool such as top, that runs in a terminal, but that keeps a cumulative total of real/user/system time used.
top already keeps a cumulative total of time spent on the cpu (user+system) I believe. That's what's listed in the TIME+ field:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 5117 michael 20 0 508m 238m 30m R 16 6.1 7:55.47 firefox 3135 root 20 0 194m 53m 14m S 8 1.4 26:37.08 X 4359 michael 20 0 117m 83m 14m S 1 2.1 17:59.34 gnome-panel
'real time' probably isn't that interesting to watch - it's just the amount of elapsed time since the process started.
If you're interested in hacking it up yourself, everything you need is in /proc/*/stat :)
Or try using ps:
ps -eo pid,user,args,etime,time,%cpu --sort %cpu watch -n1 ps -eo pid,user,args,etime,time,%cpu --sort -%cpu
(the very last line is probably closest to what you actually ask :)