You should need 512MB of RAM and I guess something like 800Mhz to 1Ghz processor is the lowest you can go on Ubuntu so it's smooth enough. If you have that I don't think there would be any trouble. If you got less RAM then that go for Xubuntu. If your processor is slower than that, there are other distributions you should be considering IMO.
And yeah the requirements slightly increase with beryl/compiz, you'd need a better processor. I think 512MB RAM should be sufficient though I have seen beryl running on 256MB RAM out of which 8MB was reserved for onboard graphics. It was an Intel 915 chipset board.