That particular 22" monitor is one of the newer ones boasting 16:9 ratio and 1920x1080 resolution for full HDTV 1080P. Nice one.
As far as your card concerns... In some games you're likely to find the difference is worth it. In others, you won't. All depends on just how well the 4870 1GB runs it. If the 4870 1GB happens to struggle in the slightest way, you shouldn't see that same struggle from a 4890. At 1920x1080, I don't think you'll see either struggle in just about anything you throw their way.
4890 is only 10% faster than 4870. Actually an Oced 4870 can perform as well as stock 4890. Now if you are not OCing I would say save your money and get the HD4870, but if you are planing to OC than 4890 is a beast. Most of the review sites were able to OC the card to 1GHz on the GPU, but then as I said is all relative. do you want to pay extra $50 for play at 57FPS instead at 52FPS. Your eyes wont notice the difference.
Playing at 1920x1080 is smooth under both cards, it all depends how much AA are you using in games.
I think some people want to have the newest baddest card. It's an expensive habit but if they can afford it more power to them.
i was thinking about the 4850X2 but idk i just said no. dont get me wrong its a wonderful card but its not for me.
im not planning to oc my card, i find it kind of pointless taking some risk to get barging rights and make the card a lil faster