the 5970 definitely blows it away but the 8800gts won't bottleneck in except for in games when you're running maximum physx at really high resolutions where the 8800gts can't keep up (pretty sure i ran into this problem with mafia ii and my eyefinity setup where the 5970 alone would get much higher frames than with the 8800gt and physx).
as for the monitor, no you don't need a physical monitor but you do need to enable an extended desktop for the card so it thinks it is putting out a resolution to a monitor.
in case you don't know, nvidia has their drivers set so that they won't work with ati cards. for setting it up, on here i'll just say to use the 257.15 drivers since they don't have the ati compatibility problems (nvidia must have forgotten to add that). just check google for the other ways

to get the drivers working.
since you already have the card and the hardware (although what power supply do you have?) i'd definitely put the card in and get it set up (easy to take it out and remove drivers too). you got nothing to lose in trying it. if you really like it but find it bottlenecks, then maybe consider upgrading the physx card to something like a gtx 250 which should easily handle any physx work you throw at it.
oh, and you can't pm until 100 posts
