Inches are not important, resolution is. Check the GPU charts and see how each performs at its (your new monitor) native resolution. I'm guessing the 4870 would be better, despite having less memory (512 v. 1024 but DDR3 v. DDR5). The 4870 1GB might be your best bet, if you can swing it. I think they can be had for $20-$40 more than a 4870 512.