A 4870 is massive overkill for that resolution, onboard graphics can handle that resolution pretty well, i wouldnt have gone for anything higher than a 4650 as that should easily max stuff out at that resolution, getting a 4870 without plans to upgrade the monitor is wasting money, i dont care what you use the system for that monitor wont let a 4870 come anywhere close to its full potential.
sometimes it is not about future proof. at that resolution you might want to look at your CPU as well. at your current resolution your CPU might not be able to keep with your GPU hence you won't see it's true potential. and i know some people don't believe this future proof thing. in my opinion it is better to have balance system so you can get all the worth you spend your money on.
yeah well that gpu will be more than enough for that res, I run nearly all modern games at that res with a 256mb 8600gts... save your money for a future upgrade and enjoy your excellent performance on a small monitor