This upgrade will cost you around $270. If a 9500 is your only option and you can get it extremely cheaply $10-20, you'll see some kind of improvement, but nothing too big.
I know you're trying to get every last bit out of the system as possible, but the other thing to consider, too, is Bad Company 2 takes advantage of modern processors by utilizing every thread you have available. It really requires a lot to run this maxed out. With the 560 ti, you'd be able to take this and put it into a new system at some point as well.
That is what I have correct...not sure what the sockets refer to.
Well , that's an XPS 410, problem is it could have come with anyone of a bunch of different chips from P4 2.8 to Pentium D to Core2Quad Q6600.
You said P4, what you have will make a difference in what is reccomended.