MikeJ:
I can help very little, I know very little of video cards.
I don't know what Riva turner does, if its speeds the clock or does another thing.
Overclocking can make a hardware run faster, but not always works.
The speed of the hardware depend on a clock, faster the clock faster the hardware. Overclocking makes the clock run faster beyond fabricant's specification and can make the hardware fail or not.
If something is specified to have 100 MHz max, but it doesn't mean that with 101 MHz will not work, it means that the fabricant garantee you that it will work up too 100 MHz, above 100 MHz the fabricant is not more responsable.
How much above 100 MHz?, well it will depend of the quality of the component and luck. Probably it will work at 110 MHz, it's only 10% above, probably it will not work at 120 MHz, but if you have luck it will work. The same model, the same version, one will work more above, other less.
The only way to know is to experiment, if it doesn't work or crash too much, use less overclocking.
Anyway is not very safe to overclock too much, even when application doesn't crash. Temperature!!!
Temperature depend on the clock, faster the clock more hot becomes the component, so exist the risk of burning the component..
You can touch the all the components of the hardware. Components can be hot but not enough to cause harm to your fingers, if something is at that level, reduce the overclocking, put another fan. If you can touch without problem then all is fine.
If you are able to run games without problem then Riva turner is ok (check the temperature). I think that the problem of Poser 7 is that is full of bugs, why should a video card make crash an application?

Stupidity also evolves!