Toss the 8600 after you get the 5770, it not going to work together, also make sure to delete all Nvidia drivers before you install the ATI card.
With a 5770 you can play HD no problem.
You dont need a monitor with mini displayport, the make active display post adapters so you can use eyefinity with any monitor. But you need a 3rd monitor if going that route. With only 2 monitors you wont have any issues.
who tells u that ppl are using ati card and nvidia card for physx.do so googling before posting.
Well if you looked on google you will see more problems using ati and nvidia, then you see success stories. Not to mention Nvidia disabled it to work on ATI without a work around, so its a no brainer that there is no official support, and hardly any game has physix anyhow, so basically for a simple user such as the OP, its a giant waste of time, that he would likely never ever see any benefit if he did get it working.
With XP, there was a requirement that there could be only one graphics driver, negating the possibility of using different type cards. I think that restriction is no longer there with windows-7, and possibly vista.
For certain, you could add something like a Nvidia GTS250 which is just a notch below the 5770 in gaming, but probably adequate for movies. It is a bit cheaper.
What PSU do you have? Is it adequate for a second card?