At that resolution, 4870 is enough to pull over 40fps (very playable) on almost all of present games. 4850x2 would perform 20-70% more subjective to game type. But as you are slow upgrader as seems from your p4 agp system, 4850x2 would suit you more than others. But if you're upgrading to crossfire motherboard, the best option is to get 4870 now and add another one later when their prices fall to like 130-ish, guess by this fall.
HD4870 1GB would be good... (HD4850X2 is a good card, but with some driver problems or even cannot manage to provide performance in those application that have no CrossFire support...)
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