I have a 24inch monitor that I intend to play 1920x1080p, but I noticed some lag today trying to play AmericanMcGee alice with my computer that only has a 4870 512mb card.
I thought about getting another card to crossfire with. I saw some cheap 4870 1gb for $100 and thought about picking it up. Would the 2 combined let me play max graphic settings without any studder or problems or should I just get something else for 1080p?
While the 512MB and 1GB cards will CrossFire, only half of the 1GB card's memory will actually be used. CrossFiring doesn't combine memory, it shares it, and can only share an amount equal-to the capacity of the card which has the least. (That was a mouthful, no?)
I tend to wonder if perhaps such a resolution is just too large for a 512MB card. Considering the game's minimum specs, it should run smoothly on that card. (That is, provided you're not using a weak CPU.) I've run into similar issues comparing performance numbers of a 4850 512MB to those of a 4890 1GB. At resolutions of 1680x1050 and above, the 4890 performs better, even when down-clocked to 4850 speeds. This is especially true when Anti-Aliasing is enabled.
If your 4870 is struggling due to lack of memory, then adding a 1GB card may not help much. I don't know if that game really needs more than 512MB or not at that resolution. Yes you should see a good boost in most games by having two 4870s. Even so the second 4870 will operate as though it too is a 512MB card, since it's the largest amount of memory both cards have in common, so you may be better off just getting a faster single card with more memory.