What have been said above I will not repeat.
Let me concentrate on the Physx card itself: when choosing a Physx card, it is the number of cores/shaders that is the most important factor. Nvidia says a minimum of 32 but experience posted all over the net indicates this amount is not really enough. Personally I suggest a min of 96.
A budget of US$150-200 for a Physx card is really a lot.
Pushing to the upper limit, a Palit GTX560 Ti 1GB can be bought at US$210. It packs 384 shaders.
Next one down, at around US$180, is EVGA GTX465 1GB which sports 352 shaders.
Then, there is a Gigabyte GTX460 1GB (336 shaders) at around US$151.
But, as VRAM is not an important factor, a Galaxy GTX460 768MB (with 336 shaders) is now selling at US$95 (after MIR) which is well below your budget.
(All prices are taken from Newegg)
Besides 'need', I agree that there are other reasons to have a Physx card. If you have one or more of these reasons, this Galaxy GTX460 768MB
is what I suggest to go along with your GTX580 SLI.
Edit Reminder: Adding a Physx card will bring in other considerations as I see that you are building a new rig. For examples, the PCIe lane layouts of the mobo, thermal management (not only the heat exhaust by the Physx card but also its possibly blocking the intake fan(s) of one of the GTX580...unless you go water cooling...the case...etc...etc)Edited by windfire - 5/8/11 at 6:21pm