First of all, sorry for my English ....
I`ve a simple dude, My PC is a Pentium IV s.775 3.06Ghz, 1Mb Cache, RAM 2Gb DDR2 533Mhz, Intel Mobo (chipset ATI x300), I want to know which VGA to buy for running Medieval 2: Total War, Empire: Total War, Crysis and CODs.
My brother has a P4 3.4GHz, 2GB of RAM, and an 8600GT and runs Medieval 2: Total War just fine. He hasn't tried any of those other games. I wouldn't invest a lot of money in a video card for that computer, because all of the other pieces are getting old. I would suggest something cheap like a Radeon 4670. You won't be able to play those games on the highest settings, but I bet they will be playable.
When I played Medieval 2 I had an 8600GT, 2GB of RAM and an Athlon 64 X2 4200+. If I remember correctly, everything in Medieval 2 ran fine at 1680x1050.
I've since upgraded my computer to an X2 5200+ and the 4670. I played Fallout 3 and Far Cry 2 at 1680x1050 with that setup. The 4670 made a very noticeable performance improvement.
It really depends on how much money you want to put into your PC. For $50-$75 I think you will see a very nice improvement with a 4670. For any amount of money over that I would look into getting a new PC.
I guess it mostly depends on personal preferences. If you only play games a few hours a week and most of that is Medieval 2, then an upgrade to your old system is fine. If you want to spend 40 hours a week playing games and want to run Crysis on high settings, then spending the money on a new system makes sense.
As far as waiting goes, there is always something on the horizon, and parts that are expensive today are almost always cheaper a few months down the road. The big problem is that reasoning means you are always waiting for something and you never get a new computer!
I think you will be able to play Medieval 2 at full settings at 1280x1024 with your PC and a 4670.
I don't know anything about using an Nvidia card for physics, but if you want to go that route you should probably look at a 9600GSO or 9600GT.