A high end card is an advantage.
If you take the time to read the many previous posts in this forum you will find plenty of information on system specifications, and what kind of performance you should expect.
The amount of information you have supplied is not really enough to give you a sensible answer.
As a help you could post up more specific detail like, what resolution you are running at, what settings you are using under graphics options, etc.
Do a search of the forums and you will find loads of very helpful information on how to greatly improve your frame rates just by changing a few settings.
But regarding what graphics card you would need, the answer is the fastest you can afford. I had a 8800GTS 320M card and found performance a bit poor in Arma2, I changed it for a GTX275 OC 896M card and it runs a fair bit better, although in this game the CPU/memory seems to be a major limiting factor on performance.
No matter what graphics card you use, do not expect to ever get frame rates like you would in BF2 or even FarCry.
Oh, and another issue is that Vista performs a lot slower than XP or Win7.
Hope that helps.
