I tested out build 7077 on the last computer I owned, which had 256MB's of RAM, a 2.3GHz (Single Core) CPU, and integrated Intel graphics circa 2004/5. Effects like Flip3D and the Taskbar Previews are a bit slow, but for the most part, the OS runs just as well as XP.
I have 7068 x86 on a Dell Inspiron 5150. 3ghz cpu, 2gb ram, and an ATI Radeon Mobility 9000 GPU. It runs decent enough, but there are no supported drivers that I could find for the GPU, so Aero does not function at all.
I also have a computer with an AMD 1600+ CPU, 1gb of Memory and an Nvidia Geforce 5900 256mb GPU that I will be loading W7 x86 on tomorrow to see how it runs.