Gtx 580 as main Gts 450 as physx

Yes, slightly, if you don't mind the extra power usage, heat, and noise. If you don't have a use for it, then might as well.

Is there any card that is better then the next for dedicated physic or not

Normally the 8800/9800gt is more than enough (single slot single power connector) and am running a evga 8800gt for this use. A GTS450 has a few advantages in physx but also disadvantages.

Pros Low idles, more cache, and many other features.
Cons Higher load and the shader units are weaker and can't have differential clocks.

You can look to a cheap gtx 460 or a gtx 260 for physx and they have a Lot of power behind them as a PPU.

My max use tested thus far on my 8800gt as a PPU is 44% in fluid mark o.O
AVG 14-30% in Mafia 2 but my cpu sucks so I have to wait till monday to upgrade.

Yeah, i am thinking about having a contest and giving away the gts 450 on here neway.

Ah I see, if it is making the case very hot and there is no way of keeping it cool then give something else a try. If nothing really suits the job then go single card. For me I got lucky, it had to be single slot or it would have cost $20 just to use it by replacing my wifi card. Single power connector as it would not have been worth it to replace my 85% rated psu.

Good luck   There are a few single slot GTS 450s out now but the heat is still a drawback.