Short answer: No degradation.
While the bandwidth of pcie 2.0 is higher, current video cards do not use all of 1.0. It is only when you get into dual and triple Highest end cards does the speed limitation show up.
Even then, It is not a significant difference.
If you are interested in gaming, spend your funds on your graphic configuration and not the cpu. Sandy bridge and bulldozer will not bring much more to the table than current products if you match them with a good vga card/s.
I suggest he get a motherboard he can live with for some time up front. Sell all the old parts that can't be carried over.