agreed with the 1GB suggestion. If you aren't going to be using 3 monitors, I would go with
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121363
the Asus 5000 series cards are outstanding to say the least and very high quality. Either this or XFX, but with XFX remember you are paying for the 3 monitor support, which if you don't plan on using 3 monitors, is useless, but, because you have your 9800GTX+ in there too (which to jarlmaster, is a GTS250, so not outdated at all) you will be able to run up to 4 monitors, just not utilisise eyefininty.
Also, something which hasn't come up, can your power supply handle 2 video cards? What PSU do you have? If it isn't up to the task of powering 2 fairly power hungry cards, you will have to forget about gettings another card before you can get yourself a new power supply.
What you asked about running nvidia and ATi simultaiously, they won't run with each other like they would in SLI or crossfire, but they will run fine independantly. Your nvidia card will be able to be used for physx and the 5770, or whatever other card you choose, will be able to be used to display