True, this is totally unscientific, because the benchmarks are for different games, but I couldn't find any other useful benchmarks for the HD 3450. It's not a gaming card, so reviewers don't often bother measuring it. It's good for HTPCs because it consumes little and supports HD and can even be fanless.
Probably around the performance of a 7900.
It isn't even remotely close to a 7900. The 3450 may have DX10, but the 7900 will flatten it in gaming performance.
I'd say around the 8500GT range, maybe a bit worse, but it far excels the HD performance of the 8500GT and many other cards that are much more expensive.
I built an HTCP setup for a friend recently with that card. It has exceeded my expectations. Not only is it extremely good at outputting HD content, but it's Native HDMI is extremely nice. It also consumes very little power and it runs extremely cool.