No, it's already in there. Right click your upper panel, and pick "Add to panel". Scroll down through the stuff until you find "System Monitor", then add it. It'll be just like Windows tak manager when you have it minimized to the tray. You can also click the icon to get more detailed system information. It's an unscientific way of judging cpu load(as opposed to logging everything), but it'll give you a good idea of what's happen when your machine lags. If the display pegs, you have a cpu issue of some kind. If it's reasonable, you can look elsewhere for your problems.
I'm not a Linux guru by any stretch. I'm just trying to pry information from you to drill down to where the problem is. Hopefully someone more knowledgeable will chime in, but maybe we can figure it out. One of my machines is a 900mhz Celeron, and I'm running 10.04 on that. Aside from the craptastic SSD it runs as expected for a machine of it's specs. I will add that Firefox and Flash seem to run worse on Linux. There isn't much you can do about Flash, but you may prefer Opera or Chrome as a browser. I still mostly use Firefox, but if I want maximum sped, I use Chrome.