So I have decided to get Asus ENGTX 580 which i feel will serve me really well, but I am worried about the size of my screen. I am planing on getting a 23" but will this resolution last me in terms of gaming with my 580? or should I settle for a 21.5" so i can lower the resolution at a later stage?
23" will give you 1920*1080, I'd definetly stick with that.
If you spend $500 on a graphics card, you might as well use it, right?
You could always lower the resolution later if you had to.
Your 580 is made for higher resolution, go for 23"