In most games, the GTX650Ti Boost will slightly outperform the HD7850. There are likely some exceptions though, so check benchmarks in case any of your preferred games are among them. For certain types of non-gaming workloads (if this is at all relevant), however, the HD7850 will actually be significantly stronger (e.g. bitmining).
If you're just asking in general terms though, limited to games, then yeah GTX650Ti Boost.