Odds are most 3rd party non-exclusives will still come out on the 360, kind of like the original xbox after the 360's release, but most (if not all) of the 1st party games will most likely be appearing only on the "Durango". However, I expect the great content on XBLA (indy games and such) will mostly still be compatible.
EDIT: Anyways, you can get a Xbox 360 for like 99$ now, can't you? If it's either that computer or a 360, I'd choose a 360...
should i buy a xbox 360 now or pc with nvidia 640 n core i3???would games will stop being made on 360 after release of 720 after a yr?
If you were to buy a PC with those specs you would be able to run all games, just not ultra or high on all of them but an i3 is fine for gaming. Personally i would go for a PC because games are always going to be made on PC, you can do much more on PC and you can just upgrade your hardware if you have to. PC>Console
In all honesty, at that price point go for a G850/G860. They are a bit cheaper and perform just as well as a sandy i3 while gaming. Use the extra bit of scratch to snag a 7770/650 ti. It's not exactly a powerhouse build, but I can say from experience it does get the job done.