the gts450 would be a beast PhysX card. Probably overkill but if you got $130 to spend....why not??? LOL. The question is, what's your main GFX card? Is the gts450 stronger???
Or, if the gts450 is out of the price range.......Instead of a 8800/9800gt, I would actually try and find an 8800gts 512mb card. 128 stream processors VS 112, and will clock higher too on the shaders. It's pretty much a gts250 with a lower price tag. Used ones go for around 9800gt pricing anyway, around $60-70.
Although, most 9800gt's are single slot so if you would rather not take up so much space than it's still plenty adequate for PhysX.
Edit- Don't get the ''green'' version, yeah it doesn't require a 6 pin but it's clocked much lower and will be worse for PhysX.
BTW, who says overkill is a bad thing???
Yeah yeah, stock cooler but it's because I swap CPU's too much plus the 655k runs plenty cool even at 4.6ghz on the stock cooler. yay