The thermal paste will not keep the heatsink in place by itself, no. For big heatsinks you will have to get real fixing points for it... unless you're willing to glue the heatsink on the the CPU (I did that on a BPPC040, thermal paste alla around, cleaned up like 3*3mm in each corner and glued it at these points, obviously not recomended for expencive cpu's).
With a small heatsing&fan the thermal paste will help holding these flat against the CPU but it will slide of easily unless you keep it in place with something (on my A4000 with PPC060 I added a heatsing to the 060 and keep it in place with one plastic stripp at the top of the heatsink. Stripp goes through the holes in the board for the PPC heatsing holder and isn't tight on the 060, it just prevents it from sliding).
On my B1260 got a small heatsink, cut of some of it's gills to fit underneath the simm (added electrical tape to prevent shots on the simm) and kept it in place with thermal paste. It did slide some but it was mekanicaly pressed byt the simm so it wouldn't move much even though it sit uppside down in the machine.
