Which is best graphics card for a 27inch monitors ?

monitor size doesn't say nothing about performance, the resolution is what counts, probably 27inch monitor is 1920x1080.

Also i need to know how much you would spend in a graphics card and what is the rest of your system, and also what you plannign to do, gaming, or just movies, etc... before telling you what graphics card you need.

What your real question should be is "What graphics card is best for a certain resolution." Some monitors are 1920x1080, 1920x1200, or 2560x1440. If you are planning on gaming at 1920x1080 I would stick with a card like a Radeon HD 6850 or an Nvidia GTX 460. If you are wanting to game at a higher resolution You might start looking at Radeon HD 6950 or a GTX 570 or above. Hope this helps.

Depending on his budget... if he plan on playing at high graphics on 1920x1080 or 1920x1200 (probably on the 27") minimum i would say 560 ti or 6950 but that is relative, if he is using a dual core cpu and is not planning on buying another buying a graphics card like those ones will be a waste of money.