I would go with amd and ati. Why because you will be able to upgrade multiple times while using the same motherboard.
No you won't; the currently introduced am3+ sockets are a dead end the followers to bulldozer (next gen cpu wich will need am3+ socket) will need a new socket again
it was the case in the past of am2+ socket that was upgradeable to current phenomII cpus
AMD phenom II x6 1100T black edition thuban 3.3ghz: $229.99
the phenomII cpus have a very bad IPC (instructions per clock) and multiple cores are useless when 99% of software especially games have trouble to even use more than 2 cores
i would recommend the sandy bridge i5-2500 (if your not going to overclock) or the i5-2500k (if you want to overclock)
http://www.xbitlabs.com/articles/cpu/display/core-i5-2500-2400-2300_7.html#sect0as graphics card i would recommend the nvidia GTX560 (that good deal for the bucks) or the GTX570
if poor graphic graphic quality like flickering because of bad texture filtering no physX support and bad drivers doesn't hurt you you could get the ati 6870
i recommend to stay away from multi GPU like xfire because of micro lags and the need for driver profile fixes for games to support it